He might not have known I guess. I used to get them when I was a kid, and there were a few times I didnt notice. I just thought it was a runny nose and it didnt bother me. Most adults would probably wipe a runny nose though and realise it was blood.

Maybe he was embarassed. One time I had a guy sleeping on the floor at my house (he was just a friend), and he threw up on the floor right next to where he was sleeping and totally denied throwing up.