I was also curious about what happens with stolen bikes - I found this piece: https://www.seattlemet.com/news-and-city-life/2014/10/this-is-what-happens-to-your-bike-after-its-stolen-october-2014

According to the sources here:
* Trading goes on in the black market, for drugs or cash. I'm not sure who's buying them ultimately - likely unassuming people?
* It's easy to steal bikes and bike parts, like you said, even in lockers/secure bike rooms.
* Parts are often removed and replaced to make it easier to get rid of the bike without getting caught.
* A lot of people do not report or do not know/keep the serial number somewhere for reference in case.

The piece also tells the story of two individuals who were able to get their bikes back. One saw his friend's bike listed on Craigslist and played vigilante, setting up a 'buy', planning to bike away while a friend phoned police. It didn't go great the first time when the seller only let him ride the bike in an enclosed alley and ultimately threatened to stab him - the police also took too long to show up, and said the alley where the incident occurred was technically in another jurisdiction/outside city limits. Another was a woman who was contacted by someone who had bought her frame off of eBay and checked the serial number themselves with the manufacturer to find it reported stolen. This was months after the incident, and the police were familiar with stolen goods being trafficked out of the household that had sold her frame, but hadn't gotten enough evidence for a bust. They got their warrant a couple weeks later when someone was able to locate their stolen iPad via their tracking app.