Where do I go to find a list or collection of all the posts in Google+ that I have ever marked with the +1 button?
The +1 tab on my G+ profile shows only web sites I +1'ed and not the posts that I plussed when reading my stream.
Google+ doesn't stream the posts you have +1ed. So you can't retrieve that list.
To Google, that +1 tab is for everything you have found around web and recommended. Inside Google+, +1 for the posts are just a feature to show the post owner that you like that post. That's all.
You click on your profile picture to get to your profile, then click on "+1's" tab.