This value, in seconds, is the elapsed time since the most recent probe request was received from a detected station, effectively how stale the entry is. An admin can use it to tell whether a nearby device is still actively present or has already moved out of range. For example, an entry whose last-seen value keeps resetting to near zero indicates the device is still nearby and actively probing, whereas one climbing steadily suggests it has left the area and the entry is due for aging out.