My first thoughts are, two:
1) There obviously could be moisture running down the flue.
2) The moisture could be created by a short purge-cycle that doesen't allow for the hot combustion fumes to exit out thru the roof. The result would be the cold air in the attic would cause condensation to form on the outside and drip back down.
Or, there could have been a circus in town that had a monkey escape. The monkey climbed the roof and urinated down the chimney...
