Recently I bought a raspberry pi and some wifi adapters with monitoring mode. I installed airodump-ng to detect wifi devices in the vicinity and this seemed to be working well. Over the course of a few days I was able to collect 150 distinct mac addresses, not only from within my own home but also from the neighbours (assuming this from their wlan ssids).
I had a visitor for a couple of hours whose wifi on his smartphone was on but not connected to my own wifi network and he didn't show up on airodump-ng? I tested by disconnecting my wifi connection on my phone and I still showed on airodump; this is obviously also the case with my neighbours who have their wifi on but are not connected to my network. Now my question is: why do my neighbours show up and my visitor does not show up? What kind of devices will airodump see and which ones will it not detect?