That's it. I would like to know where the OS X Mail application store the contents of Trash.
It depends. Open preferences --> Accounts --> Mailbox behaviors.
Check the Trash settings.
If "Move deleted messages to the trashmailbox". Then your deleted messages will be moved to the trash mailbox, if not, then you might never have anything in the trashmailbox...
if "Store deleted messages on the server" is checked, then the mailbox that the trash is placed in is NOT on your computer...
So you may not have a local trash store...
Don't worry to much.
Look in User/computer name/mail folder of a backup
of one of the time machine back up files