Years back one of my favorite sci fi authors was A. Bertram Chandler. In his books there was a mythical device called a mass proximity indicator (MPI). As its name implies it can detect mass, and until recently this was not even close to a possibility. Recently the National Institute of Standards and Technology (NIST) created a clock so accurate that it will neither gain nor lose one second in some 15 billion years*–roughly the age of the universe.
It occurred to me that creating an array of these in MEMS (Micro-Electronic Mechanical System) format, might make an MPI a possibility, as each one would have a slightly different clock speed depending on the location of various masses near it.