They could be coming down the chimney if you have one, through a dryer vent or exhaust fan. They could have built the nest inside somewhere when the house was being built, went dormant for the winter and in March when the south wall warmed up, they came out of dormancy (or hatched). These are all guesses except for the chimney, that's where I found some getting in.

Paper wasps usually go dormant - or die out - in the Winter, depending on where you live. The stragglers you are finding probably did not have to travel from very far. Paper wasps will only nest close to a good source of decaying or rotting wood from which to build their hive. Like ants, they can be an indication that water is making its way into your house.

I would have someone inspect the attic or crawl space above your bedroom for any signs of dampness or wet insulation material. Do your exhaust fans vent into your attic? Sometimes, this generates enough humidity to decay the wood and attract paper wasps.