This is not a problem Nmap, but the problem of (mainly) smartphones (and similar devices, battery powered and connected via wifi).
These devices have different modes, saving battery. For this reason, it happens that sometimes they are not (for varying a brief moment) to see in the network.
If this phenomenon occurred precisely at the time of scanning, and we immediately responded to it (the event that the device is not in the network), so there would be a "false alarm".
So we choose a suitable number of tests and the event will occur only in case all of these (consecutive) tests are negative.
You can have to do two actions:
1) Create a few profiles with different settings and then assign to each device suitable profile
2) You can experiment with the values of the period and the security factor. You can find such value when detection is faster and yet sufficiently reliable.
Yamus wrote:If the default of 8 Security Factor and 30 seconds Scanning Period = 240 seconds, does this mean it performs a 30 second scan 8 times?
No. It performs scan 8 times, but scan usually takes only a few seconds. Sometimes it is less than one second.
I hope I explained it sufficiently clearly. I apologize for my English.