Most newer vehicles have anti-siphon devices that allow gas to flow into the tank, but won't allow a siphon hose to be inserted to the tank. Follow the filler hose from the gas cap to the tank, it's probably pretty long which also would/deter someone from trying to siphon your gas. Get some rubber hose from the parts store and try and insert the hose to see if you have a anti-siphon device.
'08 Greyhawk 31SS Traded in
TV 2018 F150 FX4 SC 3.5EB TP/MT