The bulkhead plenum area is the usual culprit. The passenger side
drain plug you should be able reach, but the driver's side one has brake servo covering access. Double check with a torch to see there no residual water pooling there. I used a little washing up detergent in piping hot water and a long handle paint brush to clean surfaces so water would drain properly. In the end I removed the passenger side rubber star-shaped drain grommet as it's likely to block again. Run hose in it to clear.
Again - good call regards windscreen having been replaced recently.
But having said that - as soon as you mentioned panoramic sunroof I thought sunroof cassette. I have a similar problem on my B8.5 Allroad and eventually traced leak to that. The sunroof mechanism is mounted in an integral sunroof cassette, it's that part - that I found was leaking. On my car it was running down the passenger side foot-well, so I needed to remove the glove-box, etc.
Before attempting this job to investigate, then remove power to battery (leave for 30 minutes), then take off A pillar trim off (details further on), pour warm water on roof and see if water drips appear at the headlining meets the A pillar trim as that will confirm that's the source of the leak.
There's front, side and A pillar airbags, so don't attempt without disconnecting battery first.
Next, remove interior grab handles at the front. You need a special hook tool to remove plastic wedge once screws are out, but you can improvise. Now interior passenger side and driver's sun visors. Next the rubber door weather surrounds, paying special attention as to how they fit. Lastly the A pillar trim. Under the plastic airbag badge - there is a screw which needs to be removed.
If it's like mine, then water was dripping from the sunroof cassette at the front, onto the roof lining and then down the A pillar trim which overlaps it. The A pillar trim is plastic and fabric covered so you're never going to see damp patches appear.
With everything off you can access under roof lining. Be careful and don't haul it off as it creases. You're going to refit so don't fully pull down or away from front of sunroof, just enough to access. Obviously, choose a decent day to do this, with plenty light and give yourself adequate time to do. This is a driveway or workshop job only.
You can test with warm water and run into the water channel of the open sunroof. Most will drain through sunroof drains. The front sunroof drains exit just below the door hinges, but chances are you'll see a drip start at the cassette. It can also leak at the sunroof drains (as a rubber tube attaches here). I added a micro-cable tie just in case. To rule out blocked drains - run some nylon strimmer cable through the drain holes. You'll never get it to run all the way down - but it's a start.
To fix, get some Sikaflex 291. This stuff comes in black and white and is basically a waterproof adhesive or sealant, but is specific for marine use. Dry off the area where drip appears and liberally apply Sikaflex and smooth off to around a couple of inches in diameter. Trust me, you don't want to do this job again. It cures quite quickly, so you can start reassembling within an hour.
Once everything is reassembled, look at sunroof from above. The drain channel that runs around the sunroof may even have a hairline crack in it. You can choose to run a little Sikaflex here too. I used a strip of Gorilla tape to seal. Leave for 24 hours before washing car and avoid Karcher type pressure washer near the sunroof, other than over it, not directly on top if that makes sense.
Connect battery again, hopefully no airbag lights. I did have one fault but cleared with VCDS software. Anyway, best of luck and let us know how you get on.
