No it could be yr expansion valve letting by or your expansion vessel could need replacing.Or a leak you cant see.
I take it, it is a combi boiler/system?
Correct.
1 of the above most likely then
there should be a 15mm copper pipe from the boiler that goes outside,trace this and see if it lets any water out,you might have to put a container under it to make sure its letting water by unless it drips onto a hard surface that youd be able to see