I parked my 2017 Ford Mustang in my apartment parking garage on the 4th floor for 2 days with out moving it

When I left it there I had more than half a tank of gasoline. When I returned to drive the car the gas tank was empty and the car would not start. There was no gas in there at all. Is there a mechanical answer for this?

Sure. Someone came along with a siphon and drained it from your tank.

If you have a locking gas cap, then perhaps your gas tank has a hole in it, and the gas has dripped out and evaporated.

Yes. You have a leak. Did you notice a puddle under your car and a strong odor of gasoline?

I found my car one morning with less gas than it should have. I've used a locking gas cap ever since.

I have no idea why it won't start. In this instance, try pumping the gas pedal 20 times while cranking the motor.

Maybe sensors shut off your fuel pump. There's a fuel pump in the gas tank in addition to the fuel pump at the motor.

It would take a really exceptional gas-siphoning to remove every last drop. It could have happened though.

No, it is called someone syphoned your tank to empty.

Only if the thief is a mechanic.

If there was no gas on the ground under the car, you've got a gas thief.

Gas thieves love domestic cars. Just open the door and suck away.