The “???” error happens with you lose sensor readings. You should see something like this:
You will get reading dropouts but not alert. This can happen for a number of reasons. My experience is that moisture can corrupt the contacts between the transmitter and contact and that over time it just won’t make a good connect. I also find that compression on the sensor site can cause outages, probably from not getting proper sensor readings under pressure. I have also popped in a transmitter on a sensor and failed to click both side below the tab, leaving the transmitter with an unreliable connection. These days I used little lever and listen to make sure I hear two clicks as it snaps into place.
By using skintac and additional opsite tape I significantly reduced the occurrence of ???. It still happens, but really is more of a sign that I’ve worn the sensor too long.