Car was with mechanic - let idle for a while - how to handle?

[BDC]

Idling for 1.5hrs miiiiight have burned a half gallon of gas at the most. Most cars burn less than .3 gal/hr at idle. So even in a 10gal. fillup, at 22mpg, that would be a loss of less than 1mpg. Hardly going to wreck anything. How bad is the calc average for this tank that your mechanic ruined anyways? And how much harder did he drive it than you might if you had a wild hair or a favorable onramp? I hope you enjoy your beautiful old BMWs more than your mech...

I've never discounted a tank for any reason, just left notes for this kind of thing.