A video posted by Ghosted UK on their YouTube channel shows a number of silhouettes walking through the corridors next to the entrance of the theatre.
Members of the group including Tim Johnson, Annabelle Barnes and Amanda Esherwood said they were excited by the discovery at their Paranormal event last month.
Mr Johnson, 51, from Caister, admitted he wasn't sure what the shadow figures were but has described them as "unexplained paranormal activity".
"I haven't got a clue what they are but it definitely isn't the shadow of people because you would have seen somebody walk in front of the window," he said.
"It is really exciting and we want to go back again to explore the building further.
At the minute it remains unexplained."
Mr Johnson said the group which was set-up in January 2019 uses a variety of high-tech pieces of equipment to help with their investigations.
The state-of-the-art kit includes night vision cameras, motion detectors, and voice recorders.
