Alarm system for portable item


I'm planning on creating an alarm system for a portable device.

Externally, in Home Assistant, I set an alarm to be on or off. When its state changes to armed, I want Node-Red to take the current location of the device (taken from Home Assistant) and set it as the current location / the one which is ok +- some meters around it.

When the GPS location gets out of the last message taken and therefore set as the "one which is ok", I want Node-Red to trigger the alarm.

How would I set the location till it's overwritten again? Maybe also externally in Home Assistant?

Do you have an idea on how to use node-red-node-geofence to dynamically set the geofence using the previously taken location or do you have any other idea on how to achieve that?