Subspace readings were indicators of subspace technology.
After the USS Voyager time traveled from 2373 to 1996 as a result of an encounter with the 29th century timeship Aeon, Security Chief Tuvok scanned the surface of Earth and detected low-frequency subspace readings emanating from the northern hemisphere. According to Commander Chakotay, subspace technology shouldn't exist for another 100 years or so, to which Captain Kathryn Janeway responded that it could be the Aeon.
Tuvok localized the scan for a more accurate reading and found it to be from the continent of North America, Pacific Coast, in the city of Los Angeles. This prompted Janeway to lead an away team to search for the ship's captain, Braxton. On the surface, Janeway's tricorder detected the subspace readings were coming from within a 100 meter radius of their position but she couldn't pinpoint the source.
On the beach, Tuvok detected that subspace readings were weaker there, so Tom Paris suggested they search the boardwalk. Elsewhere, Janeway detected the readings were coming from a homeless man inside a garbage can. After following the man into an alley, they discovered him to be Braxton – aged 30 years. The source of the subspace readings was a combadge inside his clothes. (VOY: "Future's End")