I get those messages when a device i own connects through wifi but I have been able to identify them; some are my TV's some are my family's devices, android and apple. My printer shows up. But they are all ones I allow except for "sneaky" wifi devices like home appliances even my weight scale. As long as you have only given your SSID passwords to your allowed users, you might not have any issue.
Give us some examples of device names you are suspicious of.