Stone fireplace appears damp

[cloud99]

I noticed last night that the floor of the stone fireplace looked to be damp. I'm not sure if it is damp or not as the stonework is cold to start with. I have a multifuel burner which hasn't been lit for a while.
The chimney can't be blocked off as the burner is in use and was only cleaned a few months ago. Does anyone have any ideas as to why the floor of the fireplace is appearing to be damp?

 

[mointainwalker]

Since the mass of the stone will retain its temperature, if there has been a warm breeze blow across it, this can give rise to condensation.

I have had this occur in an old stone barn.