The first answer would be: why do you need a dosimeter if you don't go in radiation areas ? If you don't use your dosimeter for more than one month, please return it to the service.
The technical explanation is that the dose that you read on the reader is background compensated by 2.33 µSv/day since the attribution of your dosimeter. If you don't go in radiation areas you should then read 0 mSv all the time, however your dosimeter is continuously registering a natural dose. This 2.33 µSv is a fix mean value estimated for all CERN locations. But in reality the natural background is not the same everywhere within CERN large boundaries and it fluctuates from on place to another. Should the natural background be a little higher at you location, the value read is the part of the background that has not been compensated by the automatic subtraction of 2.33 µSv/day. The value increases because the read doses are accumulating since the attribution of your dosimeter.
Should you read a value that unexpectedly increased of more than 0.2 mSv in one month, please contact the Dosimetry Service.