Several scenarios could be going on here.
This could be a dryer vent or bath fan vent, and moist air is leaking and building up under the siding, and freezing, then running out as liquid water in warmer temps.
Especially if the louvers are fuzzy or stuck, or duct is full of fuzz, so exhaust is trapped.
It could also be a leak from above, an ice dam or blocked gutter, or a window above with leakage, and water is running down inside the siding and getting out at this vent.