I check the tire pressure almost weekly in the riding season. I have a shelf in my shop where I use a large Sharpie marker to write down the air pressure for riding solo, two up, and two up with gear. If I'm carrying a load solo I make the adjustments as needed. I'm guessing that about half the time I'm adding air when it's low is likely due to the slight amount of air that escapes when I put the air gauge on. I always check the air pressure cold & prior to a ride. I have a small check list that is also written on the shelf. My memory is not as good as it once was. Is it possible that an aggressive riding style along with numerous & deep pot holes could let some air out on certain brands. The way I change front & rear tires on my No Mar due to cupping Nitrogen would have to be free.
Get a good gauge, check often, make sure you check it cold or prior to riding.