Well. If there is a car you don't know parked outside your house with an antenna on the roof, and a nervous looking dweeb with a laptop inside we don't need to many other variables to solve for X.

On the subject of police. Don't forget that a WiFi card is essentially a radio. Law Enforcement devices that can isolate and record transmissions are nothing new; WiFi specific devices probaly already exist. Also, any decent amount of forensics could determine that an intruders MAC had been on the network; even on those little cheap D-Link rigs. So, I guess it really depends on how badly they want to bust that ass.