I have been searching but it seams i am the only one with this problem. Hope somebody can lead me in the right direction... After exiting my truck and using the remote to lock the doors I go inside and later after 1 or 2 hrs my horn beeps. It doesn't do it every time and sometimes its in the middle of the night or 5 pm. only one short beep. I think I have some kind of security system because I have a flashing red light on the dash which is factory. F150 2007 4X4 4 door.

 

do you have any other fobs lying around the house? maybe one a kid could be playing with?

is it the same horn sound as when u press the lock button twice?

i would suggest (as well many others) to remove the batteries from any and all fobs u have and see if it continues to happen.

it could be a short in the fob, a weakening (fob) battery, etc.

 

The factory flashing red LED on the dash is not a security system, it is an immobilizer ( PATS ).

I take it you are not the 1st owner ? If so, does the truck have a security system installed ?

the FOB you have, it is the normal RKE fob, or a VSS-RMST fob ?
- picture of the RKE fob in the owners manual to compare it to.