Strange trailer light activity...

[TowX]

So I had the Uhaul trailer hooked up in my driveway the other night. While unloading it, I noticed that the trailer lights would flash very lightly every 15 seconds or so. I checked the truck to make sure the lights were off and they were. Doors were closed and interior lights were off too. I had to unplug the harness from the truck for fear of the battery running down. All factory wiring components.
Anyone else have this happen? It's just a data sample of 1 point, but I can't imagine it's anything to do with the trailer. Trailer doesn't even have electric brakes... has hydraulic surge brakes. Do I have a defective wiring module from BMW?