So I'm replaying the game for the I-don't-know-which time already and this time I decided to explore more.
I get to Hargreave's room, go look around the bookshelves, etc. and then I use nano vision on the nanosuit prototypes that he had there.
And check this out:
The C.E.L.L nanosuit, the one that C.E.L.L soldiers are wearing, has thermal signs. My guess is that this is a C.E.L.L soldier just positioned there and with changed info from the visor. When I got up close, the nano vision showed that he was actually pretty warm.
I believe that the devs cba to do it the right way and just put the C.E.L.L soldier in that positions. They were probably thinking like "Damn, this is such an important game moment, nobody's going to pay attention."