Visible crew/equipment: Data tells Maddox that he formally refuses to undergo the procedure, but he supports Maddox's intriguing ongoing work. After Maddox comments that Data is remarkable, Louvois takes note that Maddox referred to Data as "he" instead of "it," and when Louvois walks toward Picard the blue tape mark is visible on the floor where she stands. (00:43:00)
Visible crew/equipment: After the encounter with Borg in main engineering, when it cuts to the observation lounge with Guinan participating in the conference, at the start of the first shot you can see the shadow from the camera and crew person as it moves, at the bottom of the screen. (00:24:10)
Visible crew/equipment: In the Medical center, as the camera pulls back from Picard, who is lying on the bio bed, the camera is reflected on the panels angled toward the camera.
Visible crew/equipment: As La Forge and Crusher leave the bridge, and the camera closes in on Riker, there's a shadow of a piece of filming equipment on the wall behind and to the left of his head, at the upper right of the shot.
Visible crew/equipment: As Picard goes to sit in his seat, there's a shadow of the boom mike playing across the wall above Burke and Kolrami's heads.
Visible crew/equipment: As Kolrami talks to Picard from beside the aquarium, you can see the studio light panels reflected on the glass.
Who Watches The Watchers - S3-E4
Visible crew/equipment: Just after Nuria says "your power is truly boundless" a few seconds later a microphone can be seen dropping and quickly being raised again at the top of the window.
Visible crew/equipment: When Riker first beams down he uses a flashlight - if you look closely you can see the electrical cord that runs down his sleeve to an off-screen generator.
Yesterday's Enterprise - S3-E15
Visible crew/equipment: During the scan of the temporal rift at the beginning of the episode, the tip of the microphone briefly comes into view at the top of the screen above Captain Picard's head when Data says, "Nothing I have seen before." [Not visible in the remastered Blu-ray version].
Visible crew/equipment: While Troi tells Riker about the Captain's grueling negotiations between the Gemarians and Dachlyds, when Picard joins them on the bridge, there are three visible T-marks taped to the floor. (00:01:50)
Remember Me - S4-E5
Visible crew/equipment: When Dr. Crusher is on the Bridge all alone, right before the "universe" collapses, the boom mic is reflected in the Red Alert rectangle next to the turbolift.
Visible crew/equipment: When Riker approaches his "son" in the prison cell (right after he has discovered the first bluff) you can see the shadow of the mic boom on the back wall quickly moving from the middle to the right. (00:34:40)
Visible crew/equipment: In the scene in the cavern where Wesley fires his phaser to heat the rocks to keep him and Picard warm, Wesley's shadow is in the wrong place. The camera is in almost a direct line of sight between where the rocks are (which are supposed to be providing the light source) and Wesley with the cavern wall behind him. After Wesley fires his phaser and the room illuminates, his shadow should be either directly behind him or to the left of him, but his shadow appears to the right side of him. The only way his shadow would be there was if the light source was off camera a good distance to the left.
Half a Life - S4-E22
Visible crew/equipment: When Lwaxana and Deanna Troi are talking in Lwaxana's quarters (near the end of the episode), note the boom microphone you can see in the mirror as Lwaxana walks away from it the first time. Shadows from the boom microphone are also seen above Lwaxana's head on the wall. (00:40:20)
Visible crew/equipment: On Romulus, when Picard and Data are told, "Eat your soup, courtesy of a loyal establishment," the camera turns to face the duo dressed as Romulans, and an actor's T-mark is visible on the ground behind Data. (00:42:40)
Visible crew/equipment: When Picard, Spock, and Data have just escaped from Romulan Commander Sela, when they are walking out of the room look at the green glass ornament - you can see a reflection of a crew member. Corrected on Blu-ray. (00:41:10)
Visible crew/equipment: While Picard talks to Rasmussen in his ready room, when Rasmussen says, "I can't get quite as worked up as you," it cuts to a wide shot, and there's a white tape mark (T-mark) taped to the floor under the desk. (00:32:45)
Visible crew/equipment: Just before encountering the Lysian destroyer the boom is visible in the top left of the screen very briefly.
Visible crew/equipment: A boom Mic is visible about nine minutes in, just as Riker and Picard begin walking down the corridor talking about Lwaxana Troi getting married aboard the enterprise.
Answer: Yes. The doors to the holodecks can be locked when in use by anyone aboard the Enterprise so they couldn't be disturbed. However, high ranking officers like Captain Picard could override the doors as it's seen that overrides are in use even for the crews quarters. Even Barclay, when he's indulging in one of his fantasies could have the doors to the holodeck unlocked by an override code.