if your dad just goes along and it escalates and they get discovered the fallout could be much worse. Personally i would just tell him i know, cut out the bullshit and that if i find out or hear they have been hanging out anymore i will tell mum and everyone else you are a cheating bastard. At this stage its just kissing which probably wont result in divorce [more likely a few months on the couch] so its up to you to end it now.
Ive been caught a few times and ive had people walk in on me but iam good with the save. Usually when i come home after being out all night its like 4am, so by the time we go to sleep its maybe 5 and then my dad goes to work at 8 so ive been asleep like 3 hours, if no one else is home before he goes to work, he tells me that hes going and like ive gotta feed the dog or whatever, probably happens like a few times a fortnight. He doesnt do this like a normal person though, oh no.
This is from my view: iam sound asleep and very suddenly iam woken up by the sound of my door opening, its that abrupt[no knocking] that i think world war 3 has broken out and we have to get to an underground bunker NOW. Iam naked. The girl next to me is naked. His view is partially obscured by my chest of draws but that wont hold for very long if he steps into my room to hand me a note of where my family is or whatever. Thinking quick i "roll over" with my doona and push the girl on the floor[wall side of the bed] along with a blanket and feign confusion because ive been asleep. Best. Save.Ever.
My mates have walked in on me a few times but they were generally cool about it, quickly closed the door and didnt even make any noise.