Could be valve, porous rim or corrosion around the lip of the rim letting air out.
Easiest way to check is dunk the rim and tyre in a water bath, but if that isnt practical use a plant sprayer with a bit of fairly liquid and cover the rim and tyre. See if you can spot the bubbles.
I had the same on the rim on one of my winter tyres - except you could actually hear the air ******* out! The outer lip of the rim had become corroded, they simply polished it up in the tyre shop and it was as good as new again.
-1998 916 Spider 2.0TS Lusso-
H&S Corse stage 1.
CATCAMs stage 1 cams with Savali custom ECU remap.
305mm discs with 166 4-pot callipers.
Ragazzon exhaust mid section.
Biscione Racing 2x70mm rear section.
17" Zender Siena wheels with 3mm rear spacers.
etc etc etc!