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?