W00t! got a free 18 speed bike!

Look it's up to you what you do with this bike that obviously does or did at some point belong to someone else. Your friends advising you to scratch off the serial number are giving you poor advice and acting like punks. A simple phone call to report this bike as found to your local police department will settle the matter. Either they'll thank you and basically ignore it and you can keep the bike with a clear conscience or they'll tell you to turn it over to them in case the owner reports it stolen/lost. Some police departments will allow you to receive it back if it is not reclaimed by it's owner in a certain number of days (ask about this policy). Some keep bikes and sell them in bike auctions for charity once a year.

In the end it's up to you and what you feel is the morally correct action to take. Do think however if this was your bike, that you'd saved up for and it fell off your car, wouldn't you want some decent person to help you get it back?