Yes, you are just guessing. Even the sensors in the tires on the ground will go to sleep after a few minutes of not rotating.
My truck has not rotated a tire in 24 hours. I did a test just 5 min ago. I have a hand held tire pressure gauge. I checked one tire pressure with the guage then put the ignition to ACC and pull up the tire screen. I let out 5 psi as read on the guage then checked the screen. To my amazement the screen didn't change it's pressure indication. My oh my! I didn't start the truck or put it in run I just did the least possible to pull up the screen, the ACC position. This is interesting, thanks @DobaMark.
They must awaken somehow without movement. I was on a run and aired down, got home, but didn't want to air up. The next day I just went out tapped the start button twice (but did not start), I believe this is the "on" position, and the tire pressures were displayed. Now that could have just been the last known pressure at rotation? I set the Tire Fill Alert, without moving the vehicle for over 24 hours, and as I are up the sensors did alert me when filled. Three of the tires showed 37# and one 36#. Gave that one a little shot of extra air and it instantly read 37#. Again without moving the vehicle.
Small phone, I used to text on a flip phone.
