It was little things like changing their emergency contacts and next of kin listings on their personnel sheets, doing it quietly without telling everyone. Gradually moving things from Greg's apartment to Nick's house, until finally there was just some furniture in storage and only 1 home. A change of address, submitted quietly and no one noticed.
They didn't exclude their friends but any invitations from Greg took place at theirs with a comment like: "his is closer to the lab for getting to work with a hangover" but this didn't happen often. There were occasional scares like Nick's truck being stolen, yes it contained evidence but it also contained a tube of lube and a CD collection so diverse that it couldn't belong to one person alone.
It wasn't that they didn't want their friends to know, and they weren't really hiding it, just being discreet and hoping that even though these were people trained to notice little details and body language that it wouldn't extend to their friends. Sometimes the others shared looks as if to say, "what are we missing?" but they didn't really question why Nick spent all night in Greg's hospital room and the same the other way around, 'cause they all did it.
It was little things, little changes and little looks they thought no one else would notice.