I have recently noticed my 2006 FEH doing this when going downhill in L gear, well under 40 mph, and without anywhere close to a full charge showing on the battery display. If I keep it in D and just use the brakes, the rpm stays around 1100-1300. ICE does not shut off because it is not warmed up enough (this happens in the first 5-10 mins of driving).

I am hoping it has something to do with the cold weather in the morning not letting the engine shut off in L gear, although that doesn't explain (to me at least) why the engine would rev in L gear.