Tacoma tailgates are very easy to detach and steal. There's a simple solution for it, a rubber pipe clamp. I installed one just to make sure mine doesn't go missing as you saw, it's a super quick detachment, the video even looks like a Tacoma tailgate.
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Took this off the Tacoma forums, it's a simple fix. There are keyed hose clamps that would make it more difficult vs the flat head version, but you get the idea. You can also position the head to poke out between the bed and the gate. Might just be enough to keep your tailgate safe and for them to move to easier targets.