I've travelled many times. Never had a letter. I always say I have the sensor on, soem take me round the side, others through the scanner. Neither causes any problems, it stays working. I carry a spare boxed libre just in case but not needed it so far
My finger prick pack is always carried in my hand luggage and they've never commented or stopped me for it.
I have however been stopped for some very tiny scissors in a repair kit, and a hair brush that had metal core!!