Any connection that shows up with a lock symbol I know I can't connect to (except my own) but yes, sometimes I see them without a lock and a good, strong signals and then I see the behaviour you mention. As rr0123 says I assumed they are just networks that don't allow outside access. Others without locks I can connect to fine like the one someone rather handily has near my bus stop in the mornings

I know locally there are some connections that look open but you have to go to a special web page to enter a code before you can use it (at least I think that's how it works).