I'm familiar with Alleycat racing, and if I recall, there was a participant in a race in Chicago who died after running a red light and getting hit by a car. I wonder if that incident has affected how Alleycats organize their events.
There are several other types of urban scavenger-o type events, where maps are encouraged, but not provided.
City Solve,
Urban Dare,
Oyster,
The Great Urban Race,
Scavenger Dash,
Urban Assault, and the granddaddy of them all,
City Chase.
If you have the chance, there are also events put on by
SCVNGR, a location-based phone app startup. They have events all over the country, with jewelry stores as partners, usually called a "Diamond Dash" where it's free entry for teams of two, and the winning team wins a diamond ring worth a retail value of $10,000 or more.
Be warned, it's more of a marketing gimmick than an actual fair competition, but it's free and fun. I organized a team of avid urban orienteers, and we finished 2nd (on foot, against a lot of teams on bike) in the event in Seattle, snagging $500.
If anyone on AP is going to do one, hit me up. I have several tips and tricks for you. They've had these all over the country, but they aren't really easy to find. They usually cater to the about-to-have-a-wedding crowd (hey! that's me!).