Truck and Trailer Parked, Trailer Lights Flash

[SawyerTed]

Crazy thing has been occurring since I got the Chevrolet High Country.  When the truck is parked, locked and not running with a trailer connected, the trailer lights flash periodically.  It seems all Trailer lights blink including marker lights.

An internet search gives mixed answers.  Some say it is an anomaly and there's no fix.   Some say it is the security system preventing disconnect of the trailer without the alarm going off.  

Between pre Ian rains last night (I'm in Santee, SC) I unplugged the trailer and no alarm sounded. 

Does any have an idea what's going on?

[Southside]

No but I have a gooseneck that will do the same thing.

[moodnacreek]

No idea in H but my forklift can do that at times.

[caveman]

My truck has done that off and on for over 15 years, with or without a trailer attached.  I suspect the problem lies within the trailer brake controller.  When the brake lights are on (truck off, not pushing brake pedal), the brake controller will be lit up.  If I flick the manual lever on the controller, the lights usually go out.  It will happen intermittently.  The problem may also be moisture related.

The controller turned on the brakes a year or so ago while I was hauling a 30' GN down Buffalo Ave. in Tampa.  At first, I thought something was bad wrong as it felt I was dragging a heavy anchor.  Then I realized what was up and flicked the manual brake lever and all was well.  That is the only time I noticed it while driving.

[21incher]

My new Ranger  does basically the same thing. On mine it's a perimeter alarm that picks up people or animals  approaching the truck without a fob when it's locked. First the lights will flash trailer or not and if they touch the door  handles the lights will turn on for about  30 seconds. Walk up with a fob and the puddle lights come on also. You probably  have something similar.