No not true. The only way it would spark is if there was voltage trying to find ground OR voltage touching ground. There should be NO voltage until horn button pushed. So you need a few tests. Keep the power wire OFF horns. You have a problem, as you know.

DO these tests.
DO NOT push horn button.
DO NOT let any wires touch ground.
1. test removed horn power wire for voltage.
2. test all three relay terminals for voltage. Do not disconnect the wires from the relay
3. Advise how you tested. With want instrument. How was instrument used. Specifically.
4. Report results. carefully, accurately. Each test. Each question.
5. don't do anything not on this list
6. Take off negative battery cable when finished with tests.
7. If using a meter, set to 20VDC. Red Plug into volt jack. Black plug into common jack. Touch wire/terminal with red lead and touch black lead to chassis ground.

YOU MENTIONED MAST JACKET: What is your definition of Mast Jacket?????????

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn