I've thought about this problem and don't have a generally applicable answer.
On the one hand, I don't want someone to steal my bike or have their bike stolen.
On the other, the police are the worst and will probably make the situation worse.
So I lean towards "don't call the cops", but it feels like there should be something to do other than taking on the full risk of intervening yourself.
Well, the context used to just be there. Now it's not, and this is worse.