Cut U-Lock....Bicycle Left?
According to campus police reports (apparently backed by video surveillance), an organized gang of bike thieves operates in my area using techniques like this. They use the following system:


A few "spotters" stroll around the campus and pick out the $1500+ bikes.
An hour or so later, a different thief, dressed in coveralls etc. comes by and cuts the locks with appropriate tools. He then walks away, leaving each bike with a cut lock.
An hour or so later, a third group of thieves comes round to collect the bikes. They put away the cut locks exactly like a rider would put away a lock they own, so that it appears to onlookers that they are the owner. They then ride off.


So apparently at least some of the more sophisticated thieves will go to the trouble of separating the cutting from the theft, if they know they are under surveillance.