These interesting sensors are quite cheap, about USD$5 for an interesting little device, so I thought I'd take one apart and post a pic of what's inside.
The backside of the board shows the transmit and receive antennas - a pair of patch antennas at either side of the board.
Upon removing the can, you might be surprised that (except for microstrip structures) there are only 5 components - two ICs (presumably one is an oscillator and one is a mixer), a barium titanate DRO (dielectric resonant oscillator) and one resistor and a capacitor, and three pads for 5VDC, ground and the IF output.
The nominal frequency is 10.525 GHz, and a little grubscrew built into the shield can above the DRO "puck" allows for tuning.
Some people in the X-band amateur radio community have modified these with an SMA connector, for use as a general purpose 10 GHz local oscillator:
http://www.shfmicro.com/gunn.htm