answer:I’ll bet you “used to have” a chimney cap, and now you don’t. The storm may have blown it off, or awry. What I expect has happened is you have some water that came down the chimney and is now pooled on your damper. If you open the damper, then I expect you’d have a one-time mini-flood of all the water that has backed up there. Not more than a gallon or two, one would hope. That explains water “in the fireplace”. But for water outside the fireplace and dripping onto the mantel, I suppose your assumption of a roof / flashing issue is the most likely answer.