The first possibility is that the porcelain underneath is cracked. The loss of that much water a day is not a lot if there is a leak. A crack in the porcelain could result in a relatively slow leak. The other possibility is that there is an elongated strand of human hair or dental floss hung up in the drain. If half of the strands are sitting in the drain pipe leading from the toilet, and the other half are hanging inside the toilet bowl, but out of view, then the 5 cm (2 in.) of water loss during the day is likely to be the result of the string siphoning the water into the drain pipe. You should not have a problem with pulling the string out because your toilet is flushing well to begin with. There is not a major blockage, but just that piece of string that is locked in place. Using a simple closet auger may do the trick. If that does not work, then you will have to remove the toilet, turn it upside down, and remove the strands that way.