Spoilers if you haven't seen Gorizilla yet.
In the end, we are shown a large, old hall under Adrien's room, right? Okay. Go back to that part and listen REALLY closely to the whole showing part.
You can hear four metallic thumping sounds and then some kind of electronic or magical beeping and magic sparkle sounds.
What do you think it could mean?
The metallic thumping was someone's footsteps on the metal platform and the beep must have been...... a Miraculous running out. And then we hear a sparkly noise right? Here's my guess:
A Miraculous holder is down in that hall and he/she detransformed after walking in.
And there IS a possibility that the holder is maybe Mrs. Agreste. She may either be in captivity or be hiding there.
I have another theory. It may be Master Fu transformed. He must have a special kind of underground passage leading from his shop to that hall.
Whatever it is, SOMEONE is down there.