A subspace signature is given off by a various objects utilizing subspace that allows those objects to be detected by scanning for each object's unique imprint.
Starships
Each starship has its own unique subspace signature which is logged by Starfleet and can be determined by all Starfleet vessels.
- In 2152, the Enterprise detected the the subspace signature of a temporal time ship from the 31st century. (ENT: "Future Tense")
- Lieutenant Commander Data, in 2368 on stardate 45571.2, detected the subspace signature of the USS Essex near an unexplored moon of Mab-Bu VI. (TNG: "Power Play")
- USS Voyager's nacelles gave off a very similar subspace signature to a large space-dwelling lifeform which threatened to rob the ship of its power. (VOY: "Elogium")
Transporters
Subspace transporters could also leave a "trail" of particles after they had been used.
- Data and Geordi La Forge, in 2370 on stardate 47829.1, scanned Captain Picard's ready room to find a subspace signature on a chair which Bok had sat in beforehand that was determined to have originated from subspace particles produced by Bok's personal subspace transporter. (TNG: "Bloodlines")