As the title says, there's this water detector/alarm (~7 years old) which recently started malfunctioning.
It starts buzzing as soon as the battery is connected to it, with or without water (a short, probably).
How do I fix this?

I know that these things are pretty cheap but I'd still like to save the money and fix the one I've got.
I attached a few photos of it.

 

Start by giving the pcb a good clean.

 

Start by giving the pcb a good clean.

I gave it a good scrub with a toothbrush and isopropyl alcohol.
Seems that it did the trick, it buzzes now only in presence of water.
Thanks! Pretty stupid from me that I didn't think of it myself.