Cut U-Lock....Bicycle Left? Is this a common thing, and if so, what is the motivation behind it?

When I went to unlock my bicycle this morning, when I touched the U-Lock, it basically fell apart.  It looks like someone used a hacksaw and cut through it.  If it had been half-cut, I might think someone interrupted them (but even that is unlikely since you just need to say, 'I've lost my key!').  But in this case, it was completely cut.  I had a cable around the tires as well, but it was all connected to the U-lock; so it was completely unsecured as soon as they cut the U-lock.

This was in a parking garage with many bicycles.  

Do bicycle thieves 'prep' bicycles and then return later to steal them all at once?  I was gone for two weeks, so I have no idea how long it has been cut.  I'm mostly just scratching my head wondering why anyone would do all the work, and then leave the bicycle?
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.