That was actually one of my first thoughts too, but he did state that "She says she did not hit anything and I can't see any evidence of it." If she's being truthful, then maybe someone ran into/backed into the tailgate when she was out of the truck?

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2007 Tundra Limited Double Cab 4x4 Super White
Key FOB Tailgate Lock (courtesy of Toxarch)
OEM back-up camera w/flip down monitor
Sirius Satellite Radio
20" OEM alloy wheels
Line-X Bed liner
UWS Black Powdercoat Low Profile Toolbox
JC Whitney Black Nerf Bars
Lund Interceptor II Bug Shield
Tekonsha Voyager Brake Controller