Running a 2 stroke at idle speed produces the least complete burn.
As a result, all the unburnt gas and oil are deposited in the exhaust housing.
After a trip, or flush, all those deposits drain down to the relief hole
and thru-hub exhaust port and leak out.
That's a good reason to keep an old carpet chunk under your outboard
when it's stored in the garage. To catch the drips and prevent oil stains on your concrete.