What's wrong with just vocalizing? Something like "I have a gun. Cops are on the way. Get the F out." Seems more effective in getting the point accross than some car horn in the driveway/street/garage....
One night many moons ago in a previous house the wifey woke to some noise, and she woke me quite certain someone was in the house. I belted out something similar, not even sure if someone was in the house, but hoping I would either hear them leave, or draw them into the narrow stairwell I was covering w/ the firearm. Hearing no sounds after 2-3 minutes I concluded no-one was there, THEN cleared the house for peace of mind. If I had some car horn blaring the whole time, I'd have no way to listen for hints as to a potential intruder's location/intention.
For me, this panic button theory rates right up there w/ rape whistles and self-induced vomitting.
-rvb
ps. I would like to find the person that invented panic buttons and b-slap them. They provide no security, and are always going off when you don't want them to. I've been tempted to take my fobs apart and disable mine.
So the guy can trip over something in your house and sue you for some fake injury?