Now I'm able to list APs, and what I guessed was unfortunately right: the WiFi.scanNetworks() function only scan the APs with a SSID.
In my list, all of the AP have a isHidden=0 in WiFi.getNetworkInfo(n, ssid, encryptionType, RSSI, BSSID, channel, isHidden);
And of course there are some hidden APs, I have an android app listing them.
Is there a solution?