Hydronic boilers operate in the pressure range between 10 and 30 psi. Water feed to the piping network goes through a separate pressure reducing valve (boiler feed regulator) that regulates below system static pressure. 12 psi is a standard setting for the boiler feeds. If the boiler loses water due to small leaks/evaporation the boiler feed will open to supply make-up water. Unless the equipment has been drained for servicing, this typical make up flow event is low flow rate and low volume. The condition described may show up as an intermittent/periodic low flow event below .5 gpm. It may also be captured during a Health Check. Therefore, the call for water at 12 psi may cause a small drip to be detected, however, it is unlikely that the small drip would be alerted on a consistent basis.