How many secret entrances are there to the Jones Salvage Yard?
In "Stuttering Parrot" (pp. 40-41 of the hardback version) Robert Arthur states that there are four secret gates to the salvage yard. Unfortunately, the books only mention two by name: Green Gate One and Red Gate Rover. It's possible that Arthur was thinking of the number of entrances to Headquarters when he used the number four (but as stated in the answer regarding "Secret Four," I believe there were five entrances to Headquarters), or he may have intended to give more information in later books and either forgot about it or did not have an opportunity before his death.