Found system low on refrigerant since last tech put one pound in. Performed a full leak search with electronic leak detector and could not find any leak. Pumped down system and nitrogen tested to 400 psig and used soap bubbles on joints I could clearly see and could not hear or physically see any leak. Nitrogen pressure was not holding steady indicating there is a for sure leak on system. Put enough refrigerant in to try and hold until office can schedule an isolation leak test on system. I did not have parts on hand to begin doing an isolation test. System is operating left in the off position since homeowner is not home.