It's the phone telling you it restarted...... Not sure why this is done, but apparently it has been coded in for several models now. This is my first Samsung since the S5. It was annoying me to no end, with the symbol and setting off my notification tone when I restarted the phone... Like I didn't know I restarted the phone in the first place.
Go into Settings - Hit the magnifying glass to search- search: system ui - hit the one with the P symbol- Notifications- go down to fbe_channel_name - put it to silent and minimized. Problem solved.. No more P symbol and no more tones going off.