I'm curious about something. Don't those vent holes usually have screens on them??
Annette, that hole pattern is pretty standard on newish homes around here and they always have 1/4" screen behind them so the bees have no trouble going through. I ran into it twice last year.
there should be a top plate so they couldn't get down in the wall....
There is always a chance they went down a hole drilled for Romex or something, but let's hope not.
Were you able to get up there and look through the holes? One of those I got last year was a new swarm just several inches inside so it was pretty easy.