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I was pulling up my drive this morning when my horn started blaring non-stop.

I got out and pulled the horn relay to make it shut up. I tested the relay and determined that it was working fine, and just noticed (now that it's getting dark and I can see them) that my tail lights are also stuck on (the tail lights, not the actual brake lights which still work if I push the brake pedal). I removed the fuse for those temporarily so that my battery will still be alive tomorrow.

Is there somewhere that these circuits might be shorting? Or is the horn some sort of weird intentional side effect to let you know something else was wrong (the internet suggested this, but it seems ridiculous that they would design it this way to me... who wants the horn to be going off until you can fix another problem unless it's something dangerous that they're trying to get you out of the car for)? Or is there some other diagnostic step that I should try next?

This is all in a 2001 Honda S2000.

Update: Last night I was driving back home after dark, so I put the tail light fuse back in. When I got home I killed the power, and was about to go remove the fuse again, when I noticed the tail lights had gone out properly. I put the horn relay back in, and that seems to have started working again too. It looks like whatever the problem is, it's transient. What would a shop do to find the problem at this point?

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Though I can't prove it (I haven't been able to reproduce the problem to confirm), I think I may have found the answer to my issue (thanks to Elias' comment).

I realized recently that I only noticed that the issue had disappeared after I had installed a separate, unrelated, electrical system which required that I add a new fuse (I cheated and used one of those "add-a-circuit" fuse-splitter things). I grounded that circuit on the same bolt that the rest of the under dash fuse box is grounded to.

There's a chance that the ground was bad there, and when I added the new circuit I tightened that bolt down and fixed the grounding problem. I don't have a circuit diagram in front of me to see if this is even remotely possible (eg. if the horn is actually grounded there), but since the problem disappeared and hasn't come back, and that's the only other work I've done, I'm currently assuming that it was a grounding issue on the under-dash fuse box.