-- OK folks, let's hear your responses to this scenario:

You see a guy cutting a lock off a bike.
So YOU say "______________________________".
So HE says, "don't worry it's my own bike, I lost the key".
So then what? Maybe his story is true, maybe not.
Call the cops? OK, they'll get there after he is long gone.
What do you do in the meantime? You were on your way to SOMEplace else.
(If you physically restrain him and he's telling the truth, you can be arrested for assault.)

Fill in the blanks. Give us a realistic scenario.