The water is going somewhere, if the leak is on the boiler you will see evidence of it under the boiler, if it's going out the discharge pipe you will see it dripping from the pipe outside (if it's fitted), you can put a bottle under it to monitor it, if it turns out it's coming out of the discharge pipe then there may be issues with the boiler building up to much pressure and discharging itself for safety, if so call a engineer, if it's none of the above then the chances are you have a leak on your system pipework, which a good plumber should be able to deal with, remember to have your system water checked as refilling all the time can cause the inhibitor(if added) to be diluted or ineffective leading to dirty water which is a big problem in it's self.