It's held at the Depot at Acton, which only has a few Open Weekends each year. "The depot, which is normally closed to the public, has over 370,000 objects including 70 road and rail vehicles, signs, models, photographs, engineering drawings, uniforms and original works of art used for the Museum's celebrated poster collection."
Alternatively if you'd just like to come along to the Museum Depot it's best to book tickets in advance and call 020 7565 7298 to avoid the queues.
There's a lot of fun stuff there including rides on the Depot's miniature railway which features steam and electric trains, trips on full size heritage vehicles - including the 1950s prototype of the Routemaster bus (the RM1), and tours of the poster and artwork stores led by Museum curators.
For adults the tickets are £10.00, Seniors, £8.00, Concessions/Friends £6.00 and accompanied children under 16 & TfL staff get in free. The tickets are valid for both the 7th and 8th of March.
My friend Billy Abbott aka Cowfish is one of the Flickr Scavenger Hunt old skool crew putting the clues together for the hunt. He said "I’ve called together my elite team of scavenger hunt organisers (the ones that like trains and buses just that little bit too much, at least), and Paul, Anna and I will be putting together a list of hard clues, obscure pictures and tortured puns (the last task probably sitting on my shoulders due to the relative levels of shamelessness in our merry band - them: some dignity left, me: wearing 90s nu-meeja glasses) to puzzle and potentially confound the bunch of people that come to play." More details on his blog post.
I'm kind of looking forward to being tortured and hope to see some of you there too.