So in the Eastern wing of the Pokémon Mansion owned by Mr. Backlot in Route 212, there is a single room that a maid is guarding. Everytime you talk to her, she'll say:
I'm so sorry. Anything beyond this line is strictly off-limits. You really musn't! Mr. Backlot will be so cross with me!
I like to think that it is a room for some sort of event, but what event? Manaphy, you get it from an egg. Shaymin or Darkrai? No way.
Even if it is not an event room, what is behind that maid? I watched this video uploaded by a JP Pokétuber, which looks very convincing at first to me, but I looked up their channel and turns out they mod things a lot for Pokémon games (I recommend watching it if you understand Japanese! Its quite interesting!).
TL;DR: I'm just curious about the blocked room in the eastern wing of Pokémon Mansion. Asking for Platinum specifically.