I've seen the same behavior with the Scangauge and multiple Toyota vehicles. Sitting still, engine off but car turned on, it reads 0.03 gph. I've only seen it on Toyotas, though.
I think what's going on here is that the ScanGauge doesn't actually read the amount of fuel being used (unless they've changed something since I bought mine). Instead, they're using some algorithm to guess at it - otherwise, why would you even need to input tank size or engine type? I know mine (on an Insight) never reads lower than 0.1 gph.
Okay, here's the response I received from Linear Logic, sounds logical:
We've seen in some hybrids (mainly the Prius model) that when the engine shuts down and the vehicle is running on battery alone the ECU still reports very small values related to fuel consumption even though they really should be 0. The ScanGauge picks up these small values and converts them into GPH readings. The net effect isn't that great though since it would take an hour of driving only on battery power for the ScanGauge to think you've used 0.03 gallons. The main annoyance is probably the fact that MPG bounces around instead of indicating 9999 (essentially infinite), but unfortunately there isn't a way to adjust this error out since it's based solely on the sensor readings themselves.
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