in the galactic warehouse
Pokemon event
In Pokemon Platinum you dont get the key from a grunt you get from the detective, before getting the key you have to go talk to prof. rowan in the library .
go on Pokemon Platinum Walkthrough you'll might get the idea.
You can only get the secret key in Pokemon Platinum
you need wifi do get it!
its easy but hard
check serebii.net for info on upcoming events for Pokemon Diamond,Pearl,Platinum.
It's exclusive to Pokemon platinum version.
platinum is coming to America in 2009 ask someone in japan
Looker will meet you at the warehouse and he will have the key.
Go to www.pokemonplatinum.com and click mystery key
i think the key is "ENTER"