Plenum Cable in Ducts
Diligence is required to make sure that a non-plenum airspace stays that way. A non-plenum airspace can become a plenum airspace by accident if the ductwork is disconnected and not properly resealed.
Ductwork disconnection can also occur due to building damage, adverse environmental factors, or simply negligence.
In all such cases, eliminating unintended plenums is difficult due to the hidden nature, limited space, and difficult access of most installed drop ceilings.
For highest fire safety, it is best to assume all drop-ceilings are plenums, whether or not they are officially designated as one.