This starts out during season five, in the episode of "As you were"
"Well, looky here. I don't usually use the word delicious... but I've gotta wager this little tableau must sting a bit, eh? Me and your former? Must kill. What can I say? Girl just needs a little monster in her man" Spike said smugly as he looked at Riley.
"That's not why I'm here ... Doctor" Riley said coldly, and Spike gave him an odd slightly confused look.
"Oh god." Buffy said looking horrified and she shot Spike a pleading look, as if begging him not to have done it, not to humiliate her more.
"Here I thought we'd run you out of town, mate" Spike said sounding irritated.
Buffy got up, holding the sheet tightly, and grabbed her clothes and left the room.
"Last time I saw you, if memory serves, you were getting the juice sucked out of you by some undead ladies of very questionable reputation" Spike said tauntingly.
Riley looked away as Spike pulled on his pants.
"Now, be a good tin soldier and, uh..." Spike said trailing off as he gestured to the door.
"Where are they ... Doctor?" Riley said coldly.
"Where are what, and why do you keep calling me that?" Spike said.
"Glad to be back in Sunnydale. The locals all speak English, and I know who to beat for information. It's all brought me here" Riley said sounding almost pleased.
"Look, crew cut. She's not your bint any more. And if I can speak frankly, she always had a little thing for me, even when she was shagging you" Spike said smugly.
"Nice. That's very distracting. Now tell me, before I get unprofessional... where are the eggs, Spike?" Riley snapped as he pointed his gun at Spike.
"Eggs? You're off your nut. It must be those drugs they were keeping you on. I did warn you" Spike said.
"Okay. We can do this the hard way, or we can do this the fatal way. Where are the eggs?" Riley snapped and punched him as Buffy walked in fully clothed.
"Look, the Doctor, it can't be Spike." Buffy said,
"Look I don't have your bleeding eggs" Spike said.
"Funny, you say that like you know something about them" Riley said.
"Yeah well, I'll admit to that, no harm there. But I ain't your bleeding doctor and I don't have the bloody eggs" Spike insisted.
"You knew something about the doctor, and you did not tell me" Buffy accused.
"Well I did plan to eventually, but you said it wasn't too urgent, you did not make like it was an end of the world deal. You did not seem like you were in a hurry, so I was planning on telling you a little later, you kinda distracted me" Spike said.
"Just shut up, how do you know about the eggs?" Buffy snapped.
"Well, an associate of mine offered them to me, and wanted me to hold them for him, but I turned him down" Spike told them.
"Sure you did" Riley said.
"Look captain cardboard, I may not be a bloody saint, and I'll admit this kind of thing is my style, but I ain't completely daft, you don't bring illegal merchandise to the same place were you shag the slayer" Spike told them, and Buffy winced.
"Somehow I just can't find it in me to believe you, I'm not leaving this place till I find them, Doctor" Riley said.
"You can stop calling me that any time" Spike snapped.
"Riley this is pointless, Spike, I mean he is not good, but he doesn't do this now, not anymore" Buffy said.
"We'll see" Riley said and he headed down the ladder.
"There ain't nothing to see down here" Spike said following and gesturing to the room, where there were not any eggs; there was however a pair of black panties on the bed.
Spike reached for them and snatched them up and said "Oh luv, you left this last time, thought you might want it back. But maybe its best to leave it here, so you have a spare for the next time you can't find them, that seems to happen a lot" Spike said.
"God, just shut up Spike" Buffy pleaded.
"Fine, you're no fun" Spike said.
"We'll I'll admit, it doesn't look like they are here" Riley said.
"Thank god" Buffy said under her breath.
"Well I bloody well did tell you that, but no, of course no one listens to me" Spike said.
"Can you shut him up?" Riley asked.
"Not so far" Buffy replied.
"Wonderful" Riley muttered.
"So do tell, what kinda time table are we on now?" Spike asked.
"We?" Riley said.
"Yeah well, looks like I'm gonna be dragged into this" Spike said.
"What makes you say that" Riley asked.
"Well I was assuming you'd want me to find that associate of mine, the one who offered the eggs, but hey, you don't want my help, that's fine by me. I wouldn't mind getting a bit of sleep, I'm knackered, I had a busy night" Spike said.
"Fine, let's go. I'd like to find these things before they hatch" Riley said.
"Oh joy, this is just like old times" Spike muttered.
"Hardly" Riley muttered.
"True, this time I've got the girl" Spike said.
"I have a girl, and frankly you so much as say the wrong word to my wife I'll stake you" Riley said.
"Wife… Well that explains some things" Spike said flashing a hurt look at Buffy, clearly having figured out why she came to him.
Buffy got a guilty look on her face and she said "Spike, its not…"
"Don't, don't even lie to me. You may not like the fact, you may even hate it, but I know you. I should have figured there was a reason like that, but I was just too happy at the idea of you coming to me like that" Spike said.
"I'm sorry" Buffy whispered.
"Yeah, I know" Spike said sadly and he walked out.