Were you by any chance driving at high altitude?
The only time I've noticed my tire pressure drop is when its been setting for awhile. I never have seen it drop while moving... unless I had a leak... regardless of outside temperature. I suppose if you were moving slow enough and the outside temperature is dropping you may see some tire pressure drop.
The TPMS seems to be accurate at sea level. It will indicate lower pressure than gage pressure at altitude. You may get a sensor bad every now and again, but not all four at the same time. the only place there is anything mechanical that could stick is within the tire sensors themselves. Everything else is circuits and software.
How accurate is that mechanical gauge you are using? Do you know how its accuracy changes with temperature?
2011 Grand Cherokee Overland V8, 2009 Liberty Rocky Mt V6, 2000 Grand Cherokee Laredo I6, 1979 CJ7 I6 Quadratrac