You-(Leo) and Usagi had been dating for a while and the whole team (consisting of Raph, April, Donnie, and Mikey) had suspicions, but none had any proof except your frequent sleepovers with him, that was until now:

Donnie had walked in on you and Usagi having a make-out session on your bed. Of course Usagi nor you had any excuses "Uh- Hi, Donatello..."