My dot on the map just disappeared. Any ideas?
Check the map settings. If the sensor’s dropped offline and you haven’t checked the map in a while to notice, the grey dot will disappear after the “Reporting or modified within” time. You might also check if the sensor’s gone into setup mode again (the PurpleAir-XXXXX network is available on WiFI), which indicates that it’s lost its ability to connect to your WiFi network.
See this reply for details:
Also make sure you’re logged in. If your sensor is registered as Private, you won’t see it on the public map. Appears the map logs you out after a day or so.