Its very doubtful that it is the TPMS.
The odds of having all 4 TPM tire pressure valves leaking the same amount of air over the same period of time is beyond the realm of probability, unless Nissan has a bunch of defective valves out there.
I assume that you know never to check tire pressures when the tires are hot or have been driven on.
I would suggest that it is the weather, when cold or hot, the pressure can change as much as 5 PSI. Your best bet would be to set the pressure when the car has sat overnight and then check them a week later to see if there has been any change. Pressures should be taken with an digital tire pressure gauge, the barrel types are generally not accurate. I set the Tire pressure on my two 07 Altimas at 33 PSI, cold, sitting overnight.