During my Army Chipmunk training on one particular morning every student was ordered to report after landing. At the base of the control tower we were told a lady had telephoned to complain to the feared CFI about a low flying aircraft. She was said to have written down the serial number of the offender. We were all (or nearly all) worried as we had been in the area and might just have been lower than we ought by several hundred feet. The DCFI came out and asked if anyone wanted to admit their transgression. Nobody answered. The DCFI went back to his office and we continued to wait for what seemed like ages. Eventually we were dismissed and went to change out of flying gear. Shortly after we learned that the CFI who was not at his telephone when the lady had made the complaint phoned her up and asked for the serial number. Apparently the lady spelled out A.R.M.Y.