Ok! Yes, after some googling i also found some reference to a physical port on the device.
I was onsite about 45 minutes after the log entry and no sign of tampering.
Im not even sure if you can reach the UART on the PCB without having to unplug either power, antennas or the ethernet cable? Im monitoring the ethernet connection and 4G so i should have been notified if that was the case.
The 4G connection is behind CGNAT so it should not be reachable from the internet.
I wonder if /dev/ttyMSM0 is connectable via ssh/CLI in any way, if the device is in some way compromised you could reach it via a terminal program?
Really strange...