How about a webcam? You can get one at Walmart for like $20 and it should have a program in it that will record when it detects motion...at least, mine does (Logitech cam). It wouldn't be such a huge investment and maybe you could get to the bottom of this?

Whatever it is, I hope you can figure it out soon. I would be so sad not to be greeted by wagging tails when I got home.