The other day I turned off the engine, removed the key from the ignition and left the driver's door open. All of a sudden the horn starting sounding off and on??? I do not have a security system on this vehicle?? When I re-inserted the key and started the engine the horn stopped blowing?? This has never happened before. I also notice that when I started the truck the seat belt fastening light did not appear and I had not fastened my seatbelt.

 

You're right you don't have a factory-installed alarm - perhaps you hit the PANIC button on your keyfob?