Tap the short description to get more details about the bike, and then tap Reply to Owner and give a description of when and where you think you saw the bike. If you can snap a quick photo, that will be really helpful as well!
Trust us that we hate bike thieves as much as the next guy, but leave any confrontation to the authorities. If you see suspicious behavior, report it right away. Project 529 does not condone or recommend vigilante justice.