My understanding is that if everything is in 100% order in the braking system, the plunger should stay up and slowly drop as the linings wear. And if it does drop much whist standing, then this would indicate a fluid leak.
However, if the seal and cylinder in the brake reservoir are worn then the piston will very slowly drop as fluid passes the seal. And if there is a fluid leak, you will see fluid on the floor under the car roughly where the leak is. So don't necessarily think that plunger drop = fluid leak. Check the level first.
Hope this helps.
Chriswgawne@hotmail