Possible Clues:

• Like detectives going back for a detecting device
• Submarine detection device
• Echo sounder acronym
• It sounds like the beginning of dictation
• Now the son gets a R these days like five hundred, I see, used to be
• Early form of sonar
• Early echo sounder used to detect submarines
• Echo-sounder
• Like detectives turning up old detection method
• Early sonar, to Brits