I have seen this in my radios also. Depending on the LID you have programmed it could be responding to a ping, receive data or a profile request. Even with it set to receive only, if the LID is sent a message the radio will respond. That being said, there will usually be an indication on the display like DATA RX or PRO RX. I have not seen what happens when a ping is sent from the controller. When my radios did it there was nothing on the display, no volume adjust, nothing. I don't know if it is just a glitch in the code that it suddenly thought there was a reason to light the display or what, but the TX indicator never lit and from what i can tell nothing changed.
Im wondering, since it goes all the way back to the MRK, if it isnt just some kind of logic that was written into the first radios that they never went back and corrected or just didn't see it as a big problem. As I write this, i do know that when an accessory is attached to the side of the radio, the backlight will light up with nothing showing on the display. Perhaps static electricity present on the port would cause it to briefly think the voltage on the sense pin had changed?
Long story short, not really sure what causes it, but i have personally seen it on all my p7100's on all diff frequency bands, as well as 700p's and MRKs.