Why is my guinea pig wet?

We just got home and the whole back end of my male guinea pig is wet, and we can't work out why. We checked his bed and it doesn't look like he's had diarrhea or anything like that, and he's never got wet before when peeing so I don't think it's that. Any ideas what it could be? There is two boys in the hutch, could the other one have just peed all over him?