What was loud bang at Murrayfield Stadium?

An 'explosion' inside Murrayfield Stadium this morning was a test, the Fire Service has confirmed.
"Explosion" heard at Murrayfield. Picture; Toby Williams"Explosion" heard at Murrayfield. Picture; Toby Williams
"Explosion" heard at Murrayfield. Picture; Toby Williams

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The police said they were aware of the incident which saw the ground shake and a pall of smoke overhead.