A spokesman for the Metropolitan Police said: "The break-in occurred at the venue at some point between 3pm on July 4 and 6.45am on July 5.
"At this early stage, it is understood a bag containing mock police exhibits for a Junior Citizen project being held at the venue was taken."
There have so far been no arrests and enquiries into the incident continue.
Anyone who witnessed what happened or who has any information is urged to contact police in Hounslow by calling 101, or via Twitter: @MetCC.
Alternatively, information can be passed on anonymously by calling Crimestoppers on 0800 555 111 or visiting their website.
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