My kids are covered in unexplained bruises. Me too actually.

If they were intent on something when they were hurt, they might not have noticed. Like the time I got a massive bruise on my leg and after ages I realised it was the car door when I opened it in a mad rush - I didn't really notice it at the time since I was running late.
My boys have all sorts of knocks while running for food or toys or rough-housing, and I wait for the wail but it barely seems to register sometimes...so perhaps he got a knock while really focussed on something else?