To think this is a bit off...

[Sussexbelle73]

DS (7) has just been to his mates house for a play. Went to collect him and after ringing doorbell 3 times, DS answered door- the mates mum had gone to the shops and the dad was in bed!? So I didnt actually see an adult- aibu to be a bit pissed off?

I think so. She's only gone to the shop and an adult is in the house. It's not as if she left them alone!

quite young to be effectively home alone, though.

Depends if "gone to the shops" means popping to corner shop (back in 10 mins) (not U) or doing big family shop (an hour plus) (I think U) or gone for a mooch round clothes shopping (out all day) (definitely U).

was he invited by parents for a playdate sort of thing? Or were they out playing and decide to go back to the friends house?
There was an adult in the house that could have been woken if needed.

Oh, your OP doesn't even say that the dad was sleeping, just you were pissed off not to see an adult. yabu.

DS was invited round for a play- i think what pissed me off the most is that noone answered the door until DS came- I dont think its a 7 year olds responsibility to answer someone elses door? It could have been anyone! If the dad was sleeping then its not exactly responsible to let someone elses 7 year old child answer the door while the adult was asleep upstairs??

YANBU
sounds irresponsible on the Dad's part at least. It's possible Mum went to shops leaving Dad in charge, then he sneaked upstairs