Yeah sounds like a reasonable explanation, a tow bar relay senses the load taken by the trailers lights and some have a beeper to remind you that your towing, the wiring usually isnt in the brake light supply though, unless for some reason someones wired it into the brake line so that the beeper sounds when your braking and towing.
If theres any scotch locks in the modded wiring take them out and throw them, a long way, nasty horrid things.