[66AC]

IMO, Ron61 is spot on.
The circuit is rather simple.
Positive -> Horn Button Switch -> Horn Relay -> LOAD (Horns) -> Ground Negative

Either the Horn Switch is bad and making an ON condition in the circuit
OR
the wire from the Horn Switch to the Horn Relay and then to the LOAD (horns) is somehow intermittently touching some other positive wire causing the horns to sound briefly.

I would start with the Horn Switch.

I would remove the Horn Switch and test it with an Ohm setting on your Multimeter.

This Horn Switch is not very expensive if it is like mine. (a small push button on the dash).
So you may want to just get a new one and replace that switch.

It also could be a flaky Horn Relay? and/or the wire from the relay to the horns (33 Violet).