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.
In case you're still interested, I found a way that you can see what you're looking for. In Google+:
Hopefully there will be a way to get all +1'd posts, and in a better interface. I got the instructions from this google help page.
To see all your +1's on Google Plus posts, go to the link given below and click on +1's button: https://plus.google.com/u/0/apps/activities
To see all your +1's on external websites, click on +1's tab on your profile page. If it is not visible, then go to Google+ settings and enable the option to show this tab on your profile.
You click on your profile picture to get to your profile, then click on "+1's" tab.