Yes, someone could be connecting to your internet if there is no password or the password is weak. Maybe your modem is getting old? I'm not too tech savvy, so I may be using the wrong terms and stuff. Anyway, hope you get that resolved.
I'm actually excited for the long thanksgiving weekend because I so want to clean! Haha. I hope I don't get lazy and just want to cook and shop that weekend. Great job cleaning though! Sounds like you made great progress! Keep it up and tackle that closet!
The service provider just changed my modem a few months back, it was old they said. My wifi router is old but I tried connecting directly to the modem and the problem was still there, so the router is not problem. The service provider configures the modem, I don't know about the level of security they put in. (I'm also not tech savvy.)