Alarm, horn went off on its own

If the engine continues running for longer than two seconds, it's no longer in theft mode. You should be able to reinstall the horn fuse. One thing you might consider is low battery voltage. The alarm activated multiple times on my '93 Dynasty even though the battery was still strong enough to start the engine. New battery solved the problem.

Here's links to some articles you might find helpful:

https://www.2carpros.com/articles/how-to-replace-a-car-battery

https://www.2carpros.com/articles/how-to-reset-a-security-system