I have "Automatically retain media items when removed from project" ticked in the options for the project bay, but it doesn't seem to work properly.
1. Start a new project, with the project bay closed
2. Create a blank midi item
3. Select and delete
At this point, if I open the "Media Items" tab in the project bay, the item I deleted will not appear there. The bay will be empty. However, if I perform the same steps, but open the project bay at some point before deleting the media item, the item will be added to the list, and when deleted it will remain in the bay in the "available" state.
I might be wrong, but I don't think this is the intended behaviour.
There is also another behavioural anomaly in the project bay/media retention, which is described by xpander in the thread linked below (namely things failing to be retained if the project bay is docked in a certain way).
Tested in Reaper 4.76 and 5 pre9, OSX 64 bit.
Original thread is
here.