I think Someone Connected to My Mac Without My Permission - Help Please

Some minutes ago i was online and all of sudden the shared option shows up on my side bar with a little computer image written:
shared valued-12ef4461

what can i do to prevent this from happening again? Should i format my mac? I was using an unknown internet wi-fi that my mac found, please im really tense here, can anyone help me?

1. Stop panicking. Nothing happened.
2. You're just seeing someone else using the same network (probably using a PC ...) who didn't have their security on. You're on a Mac -- nobody (and I mean NOBODY) can see onto your Mac any further than the Public Folder (which is empty by default).
3. Stop leeching other people's internet without their permission. Them being stupid is not an excuse.

To be fair, he might not be leeching internet. I see other people's computers pop up in my network list when I'm at the library or some other public place. Sometimes they have funny things in their shared folder (and some of them clearly don't know they're sharing anything from some of the things I've found in plain sight).

Oneironaut said:

To be fair, he might not be leeching internet. I see other people's computers pop up in my network list when I'm at the library or some other public place. Sometimes they have funny things in their shared folder (and some of them clearly don't know they're sharing anything from some of the things I've found in plain sight).

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Happens to me, too. I see them listed under "Places" in finder usually.
But as it was said, as long your "Public" folder is empty, people can't see your files.

Thanks guys, yeah i wasnt trying to steal their signal or everything but i was using my internet and went down and my mac recognized the other connection automatically.
Thanks again!
and Oneironaut, it's SHE not he, lmao