my gas tank suddenly read as full... but its not

So, Often times when parked on a slight incline, my Jeep's fuel gage would get a little off, but would always correct itself once i was on flat ground. no more then 20 seconds after startup of the car.

well i said goodbye to my jeep and got the prius! but the thing is that last night i had to park it (i dont know if this ACTUALLY matters or not... just throwing it out there) on a small incline, incredibly small, like not more then 3 inch difference between the back wheels and the front.

when i got in, put my foot on the break and hit power, it showed me that i had a full tank of gas... but wait a minute....

...last night when i got home, i noticed that i still had 4 blocks left on my fuel gage, enough to last me a good 180 miles more or so before i need to refuel.... so why is it telling me that the tank is full?

Perhaps i accidentally overfilled in my last fill up? nooooo i would never top it off with the bladder system fuel tank, thats a mistake you would only make if you didnt do your research or didnt read the owners manual...

Perhaps the incline made it read off for some reason? nooooo it was a teeny tiny incline and the gas guage read full all day today, when i parked in 3 different lots, during a good 20 miles of driving.... the incline couldnt have mattered....

Maybe my family filled it up for me as a surprise! naaaaa, they dont have access to my keys, and besides, i was the last to go to bed last night and first to get up this morning, when would they have done it?

Could it be possible that some teenage hoodlum performed some misguided childish prank and put water or some other liquid into my gas tank in the middle of the night? well... if that were true, i think that it might have done something to my engine, and i would have noticed a huge difference in performance right?

any thoughts?

I brought it back to the dealership today for them to look at it, but it was too late and the mechanics were all preparing to close shop, so they wont get to it til tomorrow morning. but it has been freaking me out all day long that it reads full, but it is far from full....