Only three cplaces it would likely leak from, and the good news is that one test can isolate two of them.
Charged, cocked, no pellet, bolt closed...dump the muzzle end (without the cap that covers the fill nipple) into water. Leak that slow might take a bit of time for any bubbles to show up, but if it bubbles from the barrel, then the valve stem seal is probably leaking...if it bubbles from the nipple (either the treaded in section or the end of the nipple), then that's your problem.
OF the two, the nipple is the easy fix. There is a third way ti can commonly leak, but no easy test. Could leak past the valve body towards the rear of the rifle. No easy test for that one, but if the first test rules out the valve stem and the nipple, can be pretty sure the valve body seal is the cause.