Alram horns are a separate animal, at least on my 560. Your traffic horns should be mounted behind the grille, the alarm horns are mounted lower, underneath. I'm thinking the horns might be disconnected due to a bad switch causing a false trigger of the alarm. Check it out and see if they are disconnected, if so try reconnecting and see what happens.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.