Here's what I found with the recirc button: It was after a year of ownership while on my 8000 mile cross country trip that I finally had enough consecutive hours of seat time to notice everything.
Mind you this was all with the a/c on, not sure if it'll behave differently with it off, but I doubt it.
First off, I did not find a way to keep the recirc on ALL the time.
Starting at the other end, fresh air only... if you do want to be sure its fresh air, move the selector to defrost/windshield. This will preferentially go to fresh air, but will also turn on the a/c. You can select recirc, but it will go back to fresh after awhile (3-5 minutes?). You can hear the change by with some sound and the change of sound in the airflow. To get back to recirc, turn the recirc button off and then back on. And then do it again in a few minutes. Otherwise it will do fresh air only, except very occasionally (15 minutes or more?) it would cycle back to recirc for a minute, then back to fresh.
Now, with the vent set to dash outlets (and the a/c on) you could select fresh or recirc. Or so it seemed. I think the behaviour here was opposite of the defrost position. It would default to recirc, and then cycle to fresh air for a couple minutes every 15-20 minutes.
So, a way to keep it permanently on recirc? Not that I know of...