I have found a wasp 5 times in my small one bedroom apartment this week and I have not opened any windows and the door to my apartment is located inside. How do I determine where these wasps are coming from?
It's hard to give accurate advice without more information about the building. They've got to get in from the outside, but they can get into your room though an attic, crawlspace, etc. Go outside and look for openings. A common place is where the electrical service enters the house through an attic. If you go out in the middle of the day, you should be able to see them flying around.
The house I grew up in had a HUGE bee infestation in the attic. They got in through soffit vents. Every once and a while a couple of bees would come into my room through openings around the A/C register. No one figured out that they were coming in from the attic until honey started dripping through the drywall ceiling (my mom had first blamed me for spilling something sticky on the carpet). They had to go in and out to feed, and we did notice them outside, but we did not think they had actually nested in our house. They were much more evident outside than inside.
So, with this experience, I say take the time to observe them from the outside to determine where they are coming into the building structure, then search for nests.