Hotch already knew.
Part of his job involved ensuring that the team's personal details were up to date, and because of this Hotch tended to know more than most about the BAU's private lives. Therefore, when Reid asked for the relevant forms, he was surprised. Not surprised that Reid was involved in a serious relationship, but that he'd managed to keep it a secret for so long without anyone suspecting. Reid had explained himself steadily, without blushing or stammering, and Hotch had found himself invited to meet the mysterious Clarissa long before Strauss assigned her as a consultant.
JJ got it first.
JJ liked to think that she knew Spence, and she knew his habits, so when Strauss dumped a new Agent on them and Reid didn't make a quiet phone call at 'wheels up'; she was more than a little suspicious. Added to that, Hotch keeping Reid behind whilst Anderson determinedly didn't listen, she was pretty sure that something was going on. Spence's blush was a nice confirmation though.
Rossi had always been a people watcher.
The BAU Team didn't take kindly to intrusions, as Rossi well knew, and he was more than ready to act as a buffer for their newest acquisition. Prepared for the inevitable friction, he was pleasantly surprised to find his fear unfounded as Anderson slid into place almost seamlessly. Most of his anxiety had centred on Reid, knowing that the young genius would resent the intrusion into his field of expertise, but the pair worked well together. Too well. There was something that he couldn't quite put a name to between them, a familiarity that took time to build and that the doctors themselves were consciously keeping an eye on themselves. An idea was just beginning to form in the back of his mind by the end of the first day when they confirmed it.
Rossi happened to pass by the open door of the office that Reid and Anderson had claimed as their own, in time to confirm his suspicions. Anderson was leaning tiredly over the notebooks she was trying to translate, and Reid casually snatched the clip from her hair, letting the dark strands curl about her face. Suggestive in itself, but it was the timing that interested Rossi the most. Reid had snagged the clip just as Anderson reached up to rub at the roots of a tension headache. And neither of them had looked up. Rossi had been married three times, and he had never managed to build more than a semblance of what the young agents seemed to achieve without trying. He walked away quickly, catching JJ's eye and returning her grin.
Emily is a profiler after all.
She knew when Reid put his hand on Anderson's hip. Admittedly, she'd started off on the wrong foot, but Hotch's behaviour had confused her. When Anderson fell, Hotch had been at her side in seconds, and decidedly more hands on than he'd ever been with the rest of the team, even as Anderson brushed him off. Reid was the only one who didn't fuss, but it wasn't until later that Emily realised that his behaviour was more practised than detached. Instead, there were a myriad of little things that drew her attention, such as the way he shadowed her movements throughout the room, and always seemed ready with a feather light touch when she began to waver. All small things that Emily could have been blowing out of proportion, but when Anderson stumbled at the board, the only thing holding her up was Reid's hand on her hip, steadying her. Emily knew, and she bit back a grin as Morgan continued to flirt with the petite agent.
Garcia doesn't need to be a profiler.
Garcia glances up through her screen at Reid and Anderson as they work and suddenly sees ReidandAnderson. Reid and Anderson as in together as Spencer and Clarissa and she nearly hyperventilates as her fertile imagination fills in the blanks of Reid's recent erratic behaviour. Garcia blinks in stunned amazement and interrupts their train of thought to ask them outright. Both profilers blush vividly, and Clarissa occupies herself elsewhere as Spencer swears Garcia to secrecy for the rest of the case, before alerting the rest of the team of their new UNSUB.
Left behind as the team carries out the raid, Reid desperately tries to hush her, but Anderson just laughs, submitting to the barrage of questions that Garcia unleashes. Then her euphoria crashes as she realises the consequences their relationship could bring; super-cute genius babies or not. Anderson is a profiler, however, and she notices and she smiles tiredly because she understands.
Morgan is oblivious
On the jet home, no-one begrudges Anderson the couch as Hotch practically has to carry her up the stairs. She's embarrassed at the attention, and buries herself into the paperwork to avoid the sympathetic glances. It doesn't stop the pain, however, and Morgan is about to intervene when Reid throws down his own pen and stalks over. Without a word, he takes Anderson's files and pushes her onto her back, straightening out her legs. Anderson tries to sit up and protest, but Reid just lies down next to her, curling his lanky frame around her petite body.
Morgan turns back to Emily in disbelief as she and JJ smother giggles behind their hands. Behind them, Hotch and Rossi are exchanging money, as if they were betting on when Reid would finally give in. Morgan shakes his head at this turn of events, then smiles, pulling out his camera.