The U.S. military has been concerned over the potential hazards caused by UXO dating back to when explosives and ordnance were first developed. The awareness of UXO issues greatly increased following the end of WWII as significant numbers of training ranges were closed and the properties were transferred outside of the War Department. Evidence of interest in technologies to detect and recover UXO during this time frame is highlighted in a letter found in the National Archives (thank you Rick Stauber for the donation) from the Ordnance Department to the Chief of Ordnance within the Army. The letter dated 17 January 1946, which can be viewed using the download link below, requests information on technologies that can be used to detect UXO at practice grenade ranges. The historic letter highlights a concern that "unexploded missiles [grenades] remain under a thin layer of loose earth as a result of nearby explosions".
Although mine detectors were available at the time, they were considered by the author to be of little value for discrimation of targets due to the high amount of metal fragments in the area. The suggestion of using a long toothed harrow dragged behind a tank to uncover UXO was presented as an example of an alternate technology to detect and recover UXO. The letter requested information on technologies, which were "more simple and expeditious".
It's unclear what the response was to the letter but it's well know that many of the old ranges were transferred with little to no clearance conducted and the majority of those properties are now being addressed within the USACE Formerly Used Defense Site (FUDS) program. The letter is evidence that high clutter environments such as fragmentation rich areas have been technological challenges for UXO detection technology for the past 60+ years.