Why can other people see thank you notes we have sent on our profile page under 'Activity'?
Here's why that is the case.
In essence, a thank-you note is a comment, so it makes sense to have it listed under 'Activity' on your profile page.
Your 'Activity' tab currently shows
- the answers you've posted
- the edits you've made
- the questions you've asked
- the comments you've made
- the 'likes' you've given
- the thank-you notes you've sent
Since 'likes' are listed in your Activity tab, and since you can only send someone a thank-you note if you give them a 'like' for one of their answers first, it makes sense to have the actual thank-you note added to the tab.
Thank-you notes are public, i.e. they are visible on the profile page of the recipient, so it is assumed that you don't have a problem with them being listed in your Activity tab.
Moreover, you can always use personal notes for private stuff, so there's no reason not to have the thank-you notes you've sent listed in the Activity tab.