inside the firebox itself? Has this happened before?
If the answer to this is yes, then sometimes this occurs when there is a driving rain or rain with wind blowing that water enters the flu even with a cap and rolls down the flue to the shelf and sometimes due to capillary action will drip there instead of going back behind the box. Mine does this on special rains. I just deal with it. Its actually a defect in workmanship where the shelf meets the damper.