I didn't want to say anything about an inside job until I heard about whether or not AS&M reported a theft, but that's my gut feeling right now. I see 3 scenarios of how your boat left the yard.

1. They broke the lock or fence. - AS&M would know immediately that something was up and should have called the cops.

2. The bad guys had a key to the lock. Either stolen or given to them. They pull the boat out and lock it back up.

3. Someone let the bad guys in.

I think it would be bad business for AS&M to try to blow you off. If there is even a hint that there's someone on the inside helping out the bad guys, or that they somehow have free after-hours access, they'll probably lose more business than your boat and trailer cost combined!
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The vet said I should get my dog fixed.
I didn't realize he was broken.