Time sensor
From Tardis Wiki, the free Doctor Who reference
A time sensor was a device which, according to Jo Grant, "detect[ed] disturbances in the time field". It was most useful as a "TARDIS sniffer-outer", In particular, it was used by the Third Doctor to detect the presence of the Master's TARDIS. It was calibrated to provide distances to a disturbance in Venusian feet and miles. (TV: The Time Monster)