gt77

OK, so I checked the router physically and it is definitely a Wifi-capable router. My theory is that the wifi was somehow activated by the ISP and as the people were passing by in front of my house their phones tried to establish connection automatically.

Thank you both for the replies. I can't verify that because I have no access to the router. It's managed by ISP and I can't even establish a connection to the interface. It refuses connection on both 80 and 443 ports. Packet analysis in Wireshark revealed that the router just replies with RST,ACK TCP packet so it seems this was blocked off in the router. But I have physical access to the router and I'm going to check the model of the device and see whether it offers Wi-fi connectivity or not. I guess that if it has Wi-fi capability, it is possible this was activated by the ISP remotely. No, I have only single device and I have my Wi-fi disabled. I don't think these are "randomly generated devices" based on my changing dynamic MAC address since these are completely different devices with their own names that are quite specific (non-random) such as Martin's phone etc....

Today I ran Network Inspector which detected that roughly a month ago a lot of unknown devices (most likely smartphones judging by their names and MAC addresses) were connected to my Wired (!?) network. At first I thought I kept my Wifi turned on and these devices were just trying to connect to my wifi (it's WPA2 protected / I use strong password), but the interface offers a separate option for the Wifi network I use under the 'Networks' dropdown menu. IP Address column is blank. The router that I connect to is owned and managed by my ISP and I have no access to it.