Marshall wore a key around his neck that belonged to the closet where he and Simon stored the evidence. In the new show "Other Dimension", Mitchell is shown wearing a key that he uses to close the trunk that house the evidence.

Keys on a necklace like that - I remember that was fashion for those with both parents working where I came from (a key child that needed their own keys because no-one was home to open the door for them). I don't remember if this was the case with Marshall Wasn't that the key to the closet where he hid all the proof of the weird stuff happening?

They blew up Congress!
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