While I have no knowledge of the building at Yale, I work in a similar type of facility, and it is not inconceivable that many people in the building might have known how to access the chase.
It is very important that the plumbing and wiring systems in wet labs not go down for long periods, so in modern buildings the pipes/conduits are sometimes run through open chases inside utility closets, with only a grid cover to keep things from falling in. This lets them be serviced without having to tear out and subsequently repair walls. Storage space is often at a premium in labs, so it isn't unheard of to store unused equipment inside the utility closets, if there is room (it is normally not supposed to be done because of fire safety issues, but it happens.) If the chase was structured this way, with closet access from inside a lab or in a hallway, it is possible that workers other than maintenance personnel might have known how to get to it.
What a tragedy.....
Ugh, I am just nauseous over this.
