Some assemblies have the stopper where the tub drains from. It's a twist and fall(stopping) or twist and raise(draining).
If you tub doesn't have this then there would be a lever where the overflow is.
If you have neither, then maybe some just removed the lever from the overflow plate. They could of done this because they malfunction sometimes and keep the drain in the closed position. So someone just removes it instead of fixing it right away (How do I know this? Because I've done it before.)
Try and install a new lever and see what happens.