That looks like one of your rear sunroof drain tubes has snapped off. As with all cars fitted with factory sunroofs the sunroof seals are not 100% water tight and the sunroofs have a drain in each corner to drain off any water that bypasses the seals. Unfortunately on the Disco 2 these drain tubes are connected to the sunroof cassette by plastic adapters which have a habit of snapping off and are probably responsible for 90% of sunroof leaks on the D2. I glued my broken ones (1 on each sunroof) back on then fabricated an aluminium bracket which I glued over the top with PU sealant to support the repair and prevent a recurrence. I did this on all 8 drain tubes of the two sunroofs as preventative maintenance. Unfortunately you have to remove the entire headliner to repair these.