The internal sensors are sensors aboard starships and starbases that are used to scan for various things, including life signs, inside the vessel or structure. They perform such functions as locating various crewmembers via combadges and detecting intruders. Internal sensors are used to locate an intruder during an intruder alert situation.
In 2151, the unidentified aliens placed a device on board Enterprise which was tapped in most of the ships systems, including internal sensors and communication on all decks. (ENT: "Silent Enemy")
In 2154, Lieutenant Reed used the internal sensors of the Enterprise to locate the Vulcan bio signs of Soval but was unable to find him on board. (ENT: "Kir'Shara")
When the USS Defiant traveled back in time due to the actions of Arne Darvin, who wanted to kill James T. Kirk, Captain Sisko and the others used the internal sensors of the USS Enterprise to scan for explosives. (DS9: "Trials and Tribble-ations")
The internal sensors were used to locate Wesley Crusher when he was the only crew member, besides Data, unharmed by the game Riker brought back from Risa. The sensors were able to locate him, despite him not having his combadge on, by registering him as an unidentified heat source. (TNG: "The Game")
When strange sensor malfunctions began to appear on the USS Enterprise-D in 2368, Commander Riker ordered a level 3 diagnostic of the entire internal sensor network. (TNG: "Schisms")
In 2370, while talking to Worf in Ten Forward, Ensign Sito Jaxa mentioned that among her duties at ops was to find a lost puppy and that she used the internal sensors for this task. (TNG: "Lower Decks")
The internal sensors of Deep Space 9 were able to monitor levels of chroniton buildup during an explosive confrontation between a Prophet (in the body of Kira Nerys) and a Pah-wraith (in the body of Jake Sisko) in 2375. (DS9: "The Reckoning")