Panic/Security Alarm on Car

[RioCobra]

This has happened a couple of times, sporadically.
1997 Cobra, no aftermarket alarm.
The car is parked in the garage, unlocked, I open the driver side door, and the horn goes in alarm.
Last week I drove the car, got to my destination, opened the door to get out and the horn goes into panic mode again.
I hit the panic button on the fob and it turns off.

Any ideas on what could be causing this?
Could it be the battery in the fob?
Where can I get a replacement battery? Ford?

[tulowd]

Sounds like the button on the remote may be sticky. Pop the remote apart to see the contacts and if there is dirt in there, or anything goofy. Watch Batteries avail at walmart for cheap, so you may get lucky.

[Scrape]

sounds to me like the door pins are sticky. Check them out too.