You can only say that a glass is at a particular volume if the liquid has been super saturated. A glass filled halfway up with tap water is in fact less than half full because you can still dissolve a set amount of another material in it without changing the level of that line.
THEN you have to take into account that given enough time the water will actually leak through the glass (granted we're talking huge amounts of time) which brings up the question of how long the water has been in the glass. How thick is the glass? How much surface area of the glass is exposed to water? How much atmospheric pressure is exerted on the top of the water and how much pressure is exerted on the walls of the glass by the water?
THEN you have to calculate the humidity of the environment around the glass to determine its rate of evaporation, and under certain circumstances condensation. Does the water condensed on the outside of the glass count?
This is fun 