Unidentified motor noise coming from rear end while car is off

One possible explanation (posted in a CRV forum, but could very well apply to the Clarity):

"Part of the emissions testing on all cars is to check for fuel vapor leaks. There are many ways to do this, one of them is to seal the system and run a small vacuum pump and then use a pressure sensor to test for leaks. This has to be done well after the car is shut down and totally cooled off or else the natural pressure changes in the fuel tank will cause false readings. It is very likely you just happened to be in a quiet place when such a test occurred and you heard the pump back in the rear of the car that is used for the test."