I had this problem last year when it was wet, but then it went away of its own accord (not the first time in my Alfa!).
However, this time it looked pretty terminal - no amount of drying the car out turned the light off and I had the same problem viz the alarm going off. Guidance from this forum, although another post, suggested that just removing the switch altogether would solve the problem. Took me about half an hour tops, most of which was trying to clean the switch and the contact to get it to work properly, but the fault was with the connector slightly further back. Removed switch altogether, insulated the end of the connection and now the remote central locking and alarm work perfectly fine! I can tell visually if the bonnet's open.
Drink makes you think. Food makes you fatk.