Does anyone know how the MPG can be reset, both tank and lifetime?

I'm trying to figure out how and why this was done during a recent service visit to Sewell Lexus given that Sewell has conceded someone took my car out for a 100+ mile joy ride while it was in for service recently.

 

The tank avg resets itself when you fill up and the avg can be reset by just holding the button down while "avg" is showing.

 

Yep, just hold down the Mode button while the screen you want to reset is displayed. I do this when I fillup. You can reset several of the options this way (like avg MPH and avg MPG).