What was Gibraltar's Second World War stay-behind plan?

Did Britain plan to seal observers inside Gibraltar if Axis forces captured the Rock?

Yes. Gibraltar's tunnel network supported stay-behind concepts, although taking the heavily defended fortress would itself have been extremely difficult.

A viewer asks about the plan later known as Operation Tracer, in which observers would remain concealed inside the Rock after an enemy occupation. Gibraltar's extensive tunnels made sealed stay-behind positions possible. The answer adds an operational caveat: such a plan did not make the fortress easy to seize in the first place. Heavy coastal guns, a large garrison, and coordinated command made an Axis assault a major siege problem rather than a simple occupation.