Hi Paul

How are your tanks set up? Do you have a single tank and a UAT? Or do you have two tanks and a UAT? If you do have two tanks then are they plumbed in series there is a good chance that you will get higher idle rpm once one is empty. If it was me I would check what the PW is at when the engine is it's correct idle and fly it. Then when you land and the RPM is higher at low stick I would check if the PW is the same but you need to be quick checking because the ecu will be trying to correct the high idle so the PW will be dropping slowley. If it is then the ECU is doing its job and either the pump has loosened up during flight so supplying more fuel for a given voltage or the fuel system becomes less restrictive as the tank/s empty.

Jason