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Wikipedia: Inspector Sands
“Inspector Sands” is a code phrase used by public transport authorities in the United Kingdom, including Network Rail and London Underground, to alert staff to a fire alarm without needing to evacuate the station. The exact wording depends on the station and the nature of the incident. For example: “Would Inspector Sands please report to the operations room immediately.” or “Would Inspector Sands please report to Platform 2”.
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History
The code phrase “Mr. Sands” was used in theatres, where sand buckets were used to put out fires, as a code for fire. The word “fire” backstage would cause alarm to either performers or the audience.
 
A Glossary of Theatre Terms
copyright, Peter D. Lathan 1996-1999
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Mr Sands
Theatrical code name to warn theatre employees of a fire without frightening the audience. “Mr Sands is in the foyer” means that fire has broken out in the foyer.