I've been through Immigration/Customs in a number of US airports and the route has always been designed to prevent anyone from exiting early or hiding somewhere. For someone bent on suicide to find a conveniently open janitor's closet in this area seems to be more in the realm of Arkancide than reality.
It so happens, the Lisbon Continental flight arrived at the same time & baggage claim, so I was technically at the airport when this occurred.
FReepers might be glad to know that there were several stages of inspection & re-inspection for connecting flights.
Hundreds of people arrive at basically the same time...tough to leave the terminal, not tough to find a closet. Its not 100% escort service through customs & immigration.