Nothing to do with water pressure, and yes it could be a leak.
the water stays at a given level because of the u-bend. The shape of the bend will determine the hight of the water level.
If there is a leak in one of the joints, it will drain down to the level of the leak.
Have a good check round the toilet area for signs of water and damp.
EDIT: No, i'm not a plumber, despite what others on here will tell you.
EDIT2: Here is a diagram
The blue line is the water level. If thee is a leak anywhere below that line, the water will drain down to where that leak is. Simple gravity