Bottom line question: what is the failure mode of the rear wheel
bearings on a Focus?

Background: I have been hearing noise from the rear end of my 2001,
93,000 mile Focus for some time.  Before taking it in for a service and
MOT, I raised the rear wheels, spun them and tested them for any play.
There was no play identifiable, and they both spun without noise, except
for the nearside, which was 'catching' at a point in the revolution.
During the subsequent service, the mechanic advised it was built up
corrosion on the inside of the drum and removed this - no other problem
was identified.  I am still getting noise, a rumble, from the rear, and
am concerned that the bearing(s) may be going, and therefore wish to
know what the risk of failure is.  Does the wheel fall off, or does it
just get gradually worse to the point the failure is simple to see?

Thanks
Paul R