@Ingo:

1. Is your WPA-2 passphrase hard to crack, not dictionary based and long?
2. Are you sure WPS "easy config" feature on your WiFi access point is turned off?
3. Is the login and password to your router administration webpage different than factory default? How hard it is? Have you limited access to this page to the inside of your LAN?

These are real security concerns. Hiding SSID is placebo. Publicly available tools will reveal your hidden SSID in less than a minute.