If you have a window and a computer nearby, you could rig up a cheap webcam ($10 or so), aim it out the window, and use webcam software set to record when motion is detected (I like Yawcam - it's free - no affiliation).
DH and I did this awhile back when someone kept turning around in our yard, and we couldn't figure out who was doing it. Turns out, it was the mail lady.
The webcam wasn't anything fancy, and didn't have the greatest resolution, but for $10, it served the purpose.
