1) A puncture of some sort.
2) Some sort of manufacturing flaw.
3) The interface between the tire and the rim.
4) A valve leak.
5) The interface between the valve and the rim.
6) A leaky wheel - cracked or porous.
Try immersing the tire and wheel in water (like a kiddy pool), or spray soapy water over the area and see where the bubbles come from.
And with small leaks, this could be very difficult to detect. Patience would be a virtue.