2005 mercury sable alarm goes off when car is locked for a couple of hours.

Hello, the factory antitheft system (if so equipped, and with the horn sounding it sounds like it is) sounds like it is activating due to random false readings from the vehicle door ajar switches located on each door latch. The right way to figure it out is to hook up a diagnostic tool and see which doors show as open and which closed by monitoring body module data. If you want to guess, get some lubricating spray (WD-40 or similar, not water based) and spray each latch, then close and open the door several times to work in the lubricant. Sometimes a 'stuck' switch can loosen up and the problem may resolve. But the testing will narrow it down to the affected door or doors and that can be helpful. A switch may not be sticking but instead may still be faulty. Good luck.