Soapy water on the fill port. If it’s the same one as on the Snowpeak then it could be a little problem. When done fill it like to be bleed quickly slow bleeding seems to give me a slow leak.
mine did that and it was a leak in two place. One leak come from the my tank K-valve o-ring was bad and the second come from the brass screw ( don't know name ) on the right side. The screw had backed out from use.