Mine was front passager abs sensor got one off ebay replaced myself.
Only found it was this after getting audi specialist to scan car with odis in depth vag scanner.
Replaced a few components unnecessarily before I got car scanned.
Little tip for any1 having this issue
Remove these small rubber bungs water will flow out ...
There's 1 each side ....
not a solution but better then water building up.
Until we find the main cause.
Mine had small bit damage to abs wire behind suspension passanger side also replaced abs sensor same side soldered up brake sensors they were broken and just twisted together fault never returned....