RCW 46.37.380 calls out alarms specifically as being allowed to use a horn, and makes no mention of how often you're allowed to use a horn outside of the highway.

I'm curious which law covers it outside of that that makes it illegal to use.

Granted, I'm sure you're right if it does exist. There are some cars with ludicrously loud exhausts around here that gleefully violate RCW 46.37.390 that are ignored by cops.