Wet ash in firebox

I had not cleaned my insert firebox (Recengy i3100) sine the last burn in the winter. When I went to clean it yesterday I found all the ash was wet and muddy like. My guess is it is due to humidity and moisture coming down the chumney and building up, since the door was closed all summer and the air intake vent closed down. I guess a little rain could have also gotten in- but I think that is less likely. My recollection is that ash is very absorbant of moisture and even just a little could make it wet and muddy like. Any ideas would be appreciated.

I have a stainless steel liner and it has a nice cap that looks like it would keep any rain out very well.