Thank you to Michael Galiouris, who identified this mystery object.  It is the emergency escape hatch from a DC-10 SECURITY door.  Our door, this one, is an old type that can be breached by anyone wishing to do so.
 
(I was going make one of my lame joke attempts by listing those who might break in; Girl Scouts, Cub Scouts, Blind Quadriplegics; you know the list.  But being that it illegal in the US to use any of those terms these days, I will refrain).