The internal sensors were sensors aboard starships and starbases that were used to scan for various things, including life signs, inside the vessel or structure. They performed such functions as locating various crewmembers via combadges and detecting intruders. Internal sensors were used to locate an intruder during an intruder alert situation.
In 2151, the unidentified aliens placed a device on board Enterprise which was tapped into most of the ship's systems, including internal sensors and communications on all decks. (ENT: "Silent Enemy")
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")
Chief Miles O'Brien called the internal sensors aboard Deep Space 9 "bloody Cardassian internal sensors" because they were not sensitive enough. A recalibration of these sensors would take days. (DS9: "Q-Less")
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")
In 2372, a mutated Tom Paris caused a power outage on board the USS Voyager after firing a phaser at a port plasma conduit. Commander Chakotay reported to Captain Kathryn Janeway that he couldn't get power to the internal sensors. (VOY: "Threshold")
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 2374. (DS9: "The Reckoning")