ABS. Where to look?

[Heddy]

I could trawl through loads of ABS posts but these symptoms may be definitive. Left hand turns, moderate speed braking /slow speed braking, all normal.
Right hand turns, moderate speed braking, normal.
Right hand turns, very slow speed, ABS noise (grinding) and the ABS light pops on every time.
I'm thinking gunge/rust on a magnetic ring (or damaged) or loose sensor plug.
Now the question-which wheel? No bearing noise at all. Your thoughts please, don't want to phone a friend.
Cheers.
Just to add, after the ignition is switched off and back on again, the light goes out (self test I guess)