Via Bruce Schneier and not directly relevant to my day job of creating/preserving patent evidence but really interesting nonetheless… apparently the police can date recordings by matching up the fluctuating frequencies of power lines.
As anyone who has played with an oscilloscope knows, the 50Hz hum from mains power does show up at a low level pretty much anywhere and these make it through to the recording. It seems these fluctuations create a unique signature in time, and so they just keep a record of the power line frequency deviations over time. When they need to find out when a recording was made they can just match up the mains power fluctuations on the recording with what’s in their database.
Not majorly useful for our purposes, but a really interesting approach to authenticating evidence all the same.