I bought a WiFi extender, it plugs into the LAN and one logs in using a different name and password to main router, however unlike the main router which arrives with a random password these arrive all with the same password, and it is not easy to get access to the router to alter the password, so there are likely loads of Lidi customers all using the same password so if you see the name which Lidi WiFi extenders use, the chance is the default password will work.
As to if there is some one local who wants to play with your system is another thing, but it is human nature if some one does something to stop you doing something we look for a way around it. Noted this with Radio, tell some one go to 145.200 and no one bothers listening in, however tell some one "go to usual" and every other ham in range goes to see what you are saying.
OK I have a series of switches and TRV's which are controlled by a hub which in turn connects to router, which in turn connects to internet, and I am sure some one could break in and change the living room temperature, but why would they want to?