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It's the EGR valve working. I see this happening when I let go of the throttle on the vacuum gauge. When the EGR valve opens up, the engine lets exhaust gases in and reduces vacuum in the intake manifold. This allows the engine to turn freely and increases rpm even though no fuel is added. It's a way to save fuel and to meet emissions laws.
At least this is what I think. EGR only works when the engine is at operating temp.
Mine has been doing this as well. Auto trans and it does it at operating temp. The car as stalled twice because of this and has come close to stalling many other times. At one point the idle was around 1500 rpm until i stepped on the gas to go, very strange computer. Also get a check engine light that comes and goes, but that may be a bad gas cap. I'll have the dealer check it out next visit.
I've had it happen to me on a different car with a manual tranny. It would only happen when I shifted into neutral and coasted. But as soon as the car came to a stop, it would run at a constant RPM again, as if the speed of the car affected it.