You may not need to talk to any authorities. If it has been locked up on private property (assuming it's locked to a building, private rack, etc), then the owner of the property can determine that it has been abandoned after a period of time. Landlords deal with this all the time. In fact, if it was a tenant, their lease probably had specific clauses about this type of condition. I'd just talk to the building owner. If it has been there for months, then more than likely they'd be happy to have you cut it off so they don't have to _pay_ someone to do it later.
Cheers,
Karl