How long exactly has it been missing? Were you ever able to figure that piece out?
Who owns the place that it was stored?
He should be able to tell you exactly when it arrived and when it was discovered as missing. If not then THAT person is probably responsible for it's changing of location or at least knows who it was that came and got it (probably permitted this to happen).
Have you filed a police report? Cops can't ask anyone questions unless you have.
My feeling is that this thing has already gone to Mexico and has been stripped for parts.
Did you have insurance of any kind? Storage insurance?