How do people see what I'm posting in Facebook groups?

[dan]

I'm in several groups on Facebook - both open and closed. Sometimes I'll post something in an open group and suddenly find my friends (who aren't in this group) living my post or even commenting. For example, let's say I'm in a group about Peru and I write that I'm going there on vacation. Friends (frequently work associates) will somehow see this post even though they have nothing to do with Peru, aren't part of this group, and really should know nothing about my activity in this group.

How is this happening and, beyond remaining silent, how can I post into an open group with some small sense of privacy? I certainly understand that people can freely go to that open group and see my post....but what's leading them there?? It's driving me crazy...

Thanks!
Edit: I'm not talking about showing up in the timeline or on my own activity log. I'm talking about showing up on other people's activity log on the upper right box in the screen.

[Zoe]

It comes up on your activity log on your profile..

[Silvercat]

yes as the other answer says, it comes up on your activity log on your profile. First locate the post form your activity log in your profile(press your name on the top right). Click on the post's arrow where you can find to the top right corner. Then a menu will come, click Hide from the time line. That post is hidden to everyone. Maybe there is an option to make a selected group activity to personal. You will have to find it out