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Even without making a comment, any url that is +1'd is added to the user profile's "plusones" tab (which is public and indexable by default, but can be made private by the user).
It is not actually public. Try accessing your own [plus.google.com...] when logged out
In my case, it redirects to [plus.google.com...] (the page where my public updates are visible)
And on other people's profiles I can't even see that tab.
There might be an option somewhere to make it public but I don't see one...
That URL seems to be indexed, and it can be viewed whether logged in or out at G+.
Post in profiles & pages are confirmed but what about "comments" ?
I remember an article - may be one year old - where the author claimed that simple sharing links (via +1-button) were NOT indexed.
He talked about sharing links, not about the "plussing", as fas as I remember.
@Ann, I remember it being default for me. So maybe they changed it ? They're always tweaking.