its easy but hard
you need to go to an event
There are 2 "secret hideouts." One is in eterna city. one is in veilstone.
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 can use the statues through out the building to open stuff and there will be a place where it is all black you go walk into that then find your way through to the key
you need wifi do get it!
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.