Yesterday i noticed this guy futzing with a bike on the same rack as mine. He wasn't acting suspicious but in retrospect i did notice another bike sitting next to the rack, unlocked. He finally freed the bike from the rack and next thing i knew he was off, rolling his bike along as he rode. it wasn't until I saw him with two bikes I realized I had just watched him steal somebody's bike. After he was gone I saw the bike had been locked with a simple cable lock, which had been cut and was lying useless on the ground.
of course, the bike's owners (a couple) showed up right after. the funny thing was, they actually had two bikes locked together with the same flimsy cable, but the thief had only stolen one (both were newish road type bikes). i felt bad telling them that i basically stood around while some guy stole their bike, but i've honestly never seen it happen with my own eyes before, and it didn't register until it was too late.
now on the one hand, they were using a cable lock that could be cut with a rinky-dink wire cutter from any hardware store. the bike rack is on the side of a building, literally in the shadows (Brooklynites may be familiar with this rack on the east side of the Atlantic Terminal Mall).
but of course i feel like i should have said/done something. so finally, my question: if you think somebody is stealing a bike, what can you do or say to stop it without sounding like an accusatory jerk? i wouldn't want to accuse somebody of stealing if it was his own bike of course. this thief was pretty grim and serious but he was also an older guy who looked like he would rather not bother with the hassle of stealing if somebody had intervened. has anybody stopped a bike theft from happening like this?