@Frenchy: I wonder if what you saw might have been an example of a weakness of the "angle grinder proof" locks, which is that if the thing that the bike is locked to isn't angle grinder proof then the bike can still be removed*, and the lock removed from the bike somewhere more private. Note that the testing in the link posted by acsimpson two years ago made it clear that even the "angle grinder proof" locks could be cut through in a few minutes using a mains powered angle grinder, albeit at the cost of a few cutting discs.
I wonder how long the average Sheffield rack would last against a portable angle grinder?
I suppose there's also a chance that a savvy thief might cut the bike's frame to release it from the lock, and take the crippled bike to break for parts.
* chucked in the back of a van would presumably be a preferred modus operandi - it sounds like the miscreants you observed may not have had that luxury available.