It doesn't matter who lived in the house before YOU...if someone tapped into the house at anytime...before or after you owned it, I would power down the computers and then turn off the MAIN breaker. Your meter should not move after the shutdown. You may just have a faulty meter...
I think turning off the main breaker would be the true test to see if someone was stealing your power. Tying into the main power line would be a lot easier than tying into an existing circuit. However, I think you have a broken meter if there's anything wrong at all.