Ye Old Kissing to Avoid Discovery Trope. Is Bruce Wayne OOC? Maybe. Or you could say that since this is set before BvS, he has no reason to believe they are up to no good.

Chapter 1

Mercy's shoeless feet tiptoed across the polished hardwood floors. She stopped at an intersection of hallways. The moonlight cast the shape of the long window on the floor, and the shadow of a man cut through the center. She peeked around the corner. A stocky guard was looking out the window. His back was to her, but at a three-fourths angle. He shifted his weight on his feet, and she suspected he would turn soon. That would be her chance to cross the hallway. Sure enough, he turned slightly to his right, and she ran across.

She was almost behind the other wall when he yelled, "Hey!"

She spun around to face him. He drew his gun, but she didn't give him time to lift it. She ran forward, twisted his arm down, and punched him across the face. He grunted loudly, and she trapped him in a headlock. He struggled for a minute, then passed out. She let him slump softly to the floor, not wanting to make any more noise. She took his gun and continued down the hall.

'''''''''''''''

Lex stood near the side of the room, watching Ralph Pencham, who was reclining in a glorified armchair. For the past hour, an assembly line of socialites had rotated beside him as each cluster walked up, patted his shoulder, and wished him a swift recovery before hurrying off to indulge themselves in the open bar. The only person who spent more than one minute with him was Bruce Wayne, who was now kneeling beside him with one hand resting on his arm. Bruce was talking with him, nodding and laughing. Lex supposed it was his addiction to heroics combined with pity. The old man was going to die soon. That was why Lex needed to obtain his industrial secrets now, before the documents were bequeathed to his heir, who was much savvier about digital security than her father.

Pencham had become paranoid lately and hired guards to monitor his house, but Lex knew they would pose no threat to Mercy. In addition, they had attained a floor plan from a housekeeper, so they knew where to go for what they needed. With her skills, plus LexCorp technology, she was the perfect anti-security device.

Bruce stood up and walked over to Lex, grabbing his previously abandoned drink from a table.

"Lex," he said, nodding and taking a sip.

"Bruce."

"Where's Mercy? I haven't seen her in a while."

Lex cocked his head and stopped himself from making a hostile expression. He couldn't have Batman interfering tonight.

"Maybe because you've been busy talking to Pencham? Why, do you need her?" he asked suggestively, trying to deter him.

Bruce wasn't fazed.

"No, it's just that you two are usually joined at the hip. I've never seen you in separate rooms."

"Really? Well, she is a grown woman. Perhaps she needed the lady's room. But, you are right, it has been a while. I think I'll go look for her."

Bruce nodded again, and Lex weaved his way through the crowd. When he was in another room, he looked behind him to make sure he hadn't been followed. He scowled as he pulled his phone out of his pocket.

'''''''''''''

Mercy's smart watch vibrated. She looked down.

"The Bat suspects."

A chill ran down her spine. She and Lex were very aggravated when they arrived at the party and saw that he was there. She was an excellent fighter, but she didn't want to find out how she fared against him. The faster she got this done, the better.

She refocused on unlocking the office door. She was squatting beside it, waiting for the decryption device attached to the keyboard to give her the passcode. Two guards were bleeding out on the floor behind her. A few seconds later, the device beeped. She removed it and punched in the code. The door latch clacked as it unlocked, and she stood up and opened it. The office was old-fashioned, like Lex's. There was a fireplace on the right and a desk on the left. She closed the door behind her and walked to the desk. There was a safe beneath it. She took the knapsack off her back and pulled out a torch, welding gloves, and a face mask. Several minutes later, the safe door was open, and she pulled out a laptop. She set it on top of the desk and turned it on. The lock screen popped up, and she plugged in a drive, which hacked into the computer. She infiltrated the secure folders and downloaded the files. Then, she pocketed the drive and left the room.

Her heart thudded as she hurriedly made her way past the guards again, down to the first floor. She put her black stilettos back on, dropped the bag in the spot pre-arranged with their contact, and headed back towards the party. She intended to casually slip back into the lounge through one of the dimly-lit side hallways, but the first one she entered had Bruce Wayne at the other end of it. Thankfully, his back was turned. She bolted away as quietly as she could with her heels on. The music and chatter were hopefully loud enough to drown out the noise. She didn't know if he would come towards her or not. She pressed her back against the wall and listened for footsteps. Out the corner of her left eye, she saw movement, and her head snapped towards it.

It was Lex, and she exhaled in relief. She couldn't see his face because of the darkness, but the light behind him illuminated the edges of his dark grey suit and light brown hair. His hands were in his pockets. She stepped away from the wall, assuming he was going to lead her away from Wayne, but he walked toward her. She didn't understand what he was doing, but she stayed still and waited. As he came closer, she could make out his features. Their eyes met, and they exchanged words without speaking. He told her his ploy, and she agreed to it. It was almost an excuse, and they both knew it.

Lex stopped in front of her. He stepped closer till they were six inches apart and looked up at her, and she leaned down and pressed her lips to his. He inhaled sharply. She was immediately taken with how soft his lips were. She lingered for several seconds, then pulled away, brushing the side of her nose against his. When she went in for another, he kissed her back, lips puckering slightly. For the first time, she was delighted by their height difference. She thought about holding him by the waist. By now, she knew Wayne was in the hallway with them, silently watching, probably not believing the act.

She held Lex's face in both hands, under his jaw, and kissed him slowly and affectionately. He lifted himself up onto his tiptoes, rested his hands at her waist, and leaned closer, but his body didn't touch hers. She wondered if Wayne found this amusing. She hoped he would leave soon.

Her wish was soon granted. After about a minute, she peeked to the right and saw no one there. She continued for several seconds more, however. Their lips made soft noises as they parted and met again. Lex pulled away first, pushing off her waist, and she let go of his face. He shoved his hands back in his pockets as he turned away and strode up the hallway the way he had come. She followed close behind him with her hands folded in front of her. She couldn't deny, she could use a release after all the stress of the evening. She wondered, if she asked him, would he say yes?

She wrapped her arms around him from behind, making him freeze.

"Lex," she purred.

It was the first time she had called him by his first name.

"Would you like to continue? At home, perhaps?"

"Fuck, Mercy," he whispered harshly, "Do you have to take it so far?"

Despite his tone, he gently pulled her arms away and looked at her with eyes darkened by lust. He walked quickly through the house, guiding her by the small of her back.

If you want to get meta, you could even say that Bruce never heard her or watched them at all.