Hi @ckastely
Sensors never technically disappear from the map. They will go gray, as you have noted, which happens because a sensor has gone offline, or if all channels have become downgraded.
However, there is a map setting that affects the visibility of gray dots. By default it is set to 7 days. Which means that if a sensor has been gray for more than 7 days, you wont see it on the map. More information is available here: Does PurpleAir Remove Sensors From The Map?.
Please let me know if you have any questions.