You say you're on a retirement visa. When does it expire? A multiple reentry permit could let you continue to leave/enter Thailand until the retirement visa expires. Just an idea before you leave and invalidate what maybe a lot of time still remaining on your retirement visa...
Its a little tiny booth tucked in the corner, opposite from all the immigration officer desks. As soon as you turn the corner (towards immigration), you'll see it off by itself. You don't need to fill out or bring anything, just money. 😁