ok .. a little lecture on wifi / networking... as I understand it
WiFi use electric power, thats why devices go into a "low power mode" that does not answer traffic, but still keeps the connection to the router open: "associated" state.
FING and PING have no chance of detecting that state. Only when the phone wakes up and check with email etc it creates traffic that FING will pick up.
FINGSCAN has 4 ways of detecting presents:
1. FING ==> listens to all traffic on the network and detects new / old devices. and it then scans all known devices once a minute with PING
2. PING ==> is used in parallel to FING to detect a device going down to set device to "down" status
3. FMID ==> can be set to enable HOME / AWAY triggers ... As apple changed their security schema... does not seem to work well anymore
4. WIFI info from ASUS routers ==> detects if wifi device is "associated" if yes, it overwrites all other info and set device to "UP" status
I have observed that the iPhone 6 with IOS 8.x behaves differently when communication with the WIFI router. It disassociates some times from the router where the iphone5 was staying associated.
So if you don't have #3 and #4 you should set your "away" timeout to >15 minutes and the backHome timeout to > 5 minutes.
try and see if it helps.
Karl
the attached screen shot show the status from the wifi router the column PSM indicates the power save mode. At that point in time the iPhone 6 54:9F:13:3F:95:25 is still associated, but does not answer to pings anymore.