Confused

[nikon24]

I am confused. My 05 4.4 X runs great. Only problem I currently encounter is the low tire pressure indicator. It only happens when driving for an extended period of time. I live in Maryland, picked folks up from airport in Philly. It happened at about 40 miles into trip. Checked tire pressure and is good. Vehicle is level, I know air struts can be an issue. When I started car back up indication was gone. On the way back, after about the same distance of driving it came back on. Once again, after car was turned off, key removed, it cleared. This is not the first time this has happened. It even occured prior to me getting both front struts replaced, because one was leaking. I can drive for weeks around town, never comes on. I take a longer trip via the highway, it comes on but will reset itself after key is removed. I don't know what to make of this. Any ideas? Thank you in advance.