Tachyon scan

From Memory Alpha, the free Star Trek reference

A tachyon scan, also known as a tachyon signal involves a starship emitting a tachyon beam at a particular location in an attempt to detect cloaked vessels by finding tachyon emissions given off by a cloaking device. Although a tachyon scan would detect most vessels using a cloaking device, its range is relatively limited but can be vastly increased by setting up a tachyon detection grid. (TNG: "Redemption II", "Face of the Enemy"; DS9: "Apocalypse Rising") This technique can also be used to scan quantum singularities. (VOY: "Parallax")

However, the Remans were able to remedy tachyon scans by developing a highly-advanced cloaking device that produced no tachyon emissions, rendering the user completely undetectable to all known scanning methods. Such a cloaking device was incorporated in to their massive wardbird, the Scimitar. (Star Trek Nemesis)