Hotspots performs different activities (like witnessing, transmitting, challenging, being rewarded) over the time. HeliumGeek servers monitors these activities for your hotspot on the blockchain.
"Activity Gap" shows the difference between the height of the blockchain and the block which your hotspot was involved in any activity.
The data is fetched from the peer to peer network. As blocks are issues with certain intervals and the propagation of data takes some time, this is not perfectly immediate information but it is near-realtime information.
You can find the activities of your hotspot on the Helium Explorer page too.
Hotspots are being rewarded for the activities they perform. An inactive hotspot is not getting any rewards. You can assume more frequent the activities, more the rewards.
While the interval of some activities depend on your neighbouring nodes, challenging is kind of regularly performed activity. This means that even if you do not have many hotspots around, it is expected your hotspot perform challenge from time to time.
"Activity Gap" can be used as an early indicator for problems.
This data is presented on the hotspot screen in the mobile app. Once you activate "Instant Alerts", HeliumGeek notifies you when this gap is becoming high.
This information is based on all the blockchain activity involving the hotspot, not just PoC activities (beaconing, witnessing). These non-PoC activities include hotspot addition, location change or GRPC connection reactivation with the validator.
Some of the activities, like GRPC connection reactivation, is not shown on the Explorer activity tab too. Having a notification saying some activity happened, but not seeing any PoC activity may cause confusion. But such activities still are important for the hotspots' sanity.
During the light hotspot transition, this GRPC connection reactivation event might be happening more than usual. This should stabilize as the validators and network issues are fixed.