To be honest I think you should probably talk to your mom (not about the coming home late business because I agree that could be a lot of different things) about your dad in general.

Coming from experience, my dad was a sneaky a**hole who cheated on my mom with someone who was half his age. And unless you can get some sort of proof that he is cheating there really is no way to tell. For me, I actually met the chick and she was trying to get my dad's attention the entire time. And eventually in the company bills (he has his own company) there were suspicious items. But at the same time, it could be many things. He could be out gambling. Or hate being at home and go out drinking/playing golf/distracting himself.