Also, with an infrequently used tire, if it is completely flat, that is a sign of a problem (hole, bad valve, etc). If it is mostly flat, but still a bit spongy, then it is likely normal air loss due to leakage through the tube.

There may be an air loss curve, so a tire at 120 PSI might lose 10 PSI the first day, but the loss will slow down significantly once it reaches 80 PSI or so. Likely if one is running MTB tires at less than 50 PSI, then the air loss will be much less noticeable.

Car tires do lose air, but at a slower rate. If one leaves the car parked for a couple of years, then better check the air pressure.