I haven't seen any ways to remove/recover PIN published. I think it's because making it public is too "criminal", as it will encourage thieves to steal Garmin devices.
And how can you prove that you won't share the PIN removing procedure with bad people?
UPD: By chance I have nuvi 670 with me, and it doesn't turn on Garmin Lock without setting up the security location. So maybe try walking around your house