(covers information from several alternate timelines)
A subspace scan was a scan of the several dimensions of subspace.
Subspace scans were part of the routine bridge scans seen on monitors aboard the USS Enterprise-B. (Star Trek Generations, display graphic)
Wesley Crusher replayed Ferengi transmissions when he supposed a coded signal in one of the subspace scans. (TNG: "Ménage à Troi")
When Geordi La Forge's VISOR was somehow malfunctioning, Captain Picard ordered to run a localized subspace scan. (TNG: "Cause and Effect")
Due to Worf's concerns about the different timeline he experienced, Data initiated a subspace scan to search for temporal anomalies on and off the ship. (TNG: "Parallels")
When Captain Jean-Luc Picard traveled through time, he ordered Lieutenant Natasha Yar to perform a subspace scan of the USS Enterprise-D and its surrounding space. Yar reported no results. (TNG: "All Good Things...")
In an effort to locate Harry Kim, Captain Kathryn Janeway ordered to probe the subspace vacuoles with a high resolution subspace scan. Later subspace scans were performed with an extended search radius of ten au. (VOY: "Emanations")