4.11 Entities - Do not store entities in inaccessible areas of the map. This includes storing entities inside world props. Entities placed outside of a building must not be vertically further from the ground than a plate1x8.mdl placed vertically. This excludes areas that are accessible through an exisiting building, ladder or a staircase.
Entities shouldn't be used as a building material. They should only be used for their intentional RP purpose, excluding metal detectors, which can be used freely as long as they don't break other building rules.
It isn't allowed. You should ask big man Roy next time
PS: This rule was added around the time when the metal detector was used abusively in the disco bases.