To illustrate the concept, Brown gave them a small transponder and told them to hide it anywhere within of Army Co-operation headquarters in Bracknell. One of the TRE aircraft from RAF Christchurch would attempt to find it and fire a smoke signal within of its location. The test was carried out on 28 July 1941, and while they waited for their aircraft to arrive, another aircraft approached the hiding spot and flew around several times before flying off again. The Army suspected that they had sent this aircraft to spy out the location. Just as Brown managed to convince them they were not spies, their own aircraft, a Bristol Blenheim, arrived and fired a smoke signal only 50 yards from the transponder. To be sure, the Army was told to hide it again in another location, this time choosing to place under a tree on the lawn of their headquarters. Their Blenheim once again easily found it. It was later learned that the first aircraft was from the Fighter Interception Unit who saw the odd "blip" on their radar and decided to investigate.