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: Just before encountering the Lysian destroyer the boom is visible in the top left of the screen very briefly.
Visible crew/equipment: After Gia tells Data to remove his hood, he asks how long it will be until Skoran and the others return. Then, when Data tells her that he must hurry he takes a step forward, and a green tape mark is visible on the floor. (00:36:35)
Visible crew/equipment: After Picard, Troi, and Worf have beamed down to the planet to meet Riva, when Harmony asks if they need food or refreshments, just as Picards says, "Thank you, no," a crewmember's elbow can be seen at the left side of the screen (this one is not visible on the Netflix version). Then when Troi says, "Your method of communication is elegant and quite beautiful," a crewmember (likely the same one as earlier) can be seen at the left side of the screen. (00:07:20 - 00:10:40)
Visible crew/equipment: In the scene where Geordi and the Captain are in engineering trying to close the subspace hole, you can see a silhouette of the camera in the reflection of the warp core blue lights in the window between the warp core and the console they are working from.
Visible crew/equipment: In the scene where Picard comes to engineering to ask Laforge if he has been successful in locating Riker's homing beacon, you can see the blue light of the warp core reflected in the window. Outlined in that reflection, near Geordi's chest, you can see a studio spotlight.
Visible crew/equipment: When Deanna and Riker stop at the turbolift to discuss Data, before the doors open to reveal Data, in the shot facing Deanna the reflection of crew movement is visible on the trim's surface, just over the turbolift.
Visible crew/equipment: In the scene where the Enterprise encounters the Ferengi transport ship, the boom microphone is visible behind Commander Riker just as Data sits down at the controls.
Encounter at Farpoint (1) - S1-E1
Visible crew/equipment: On the very beginning of the second shot of the episode (the shot in engineering showing Picard walking around), the camera seems to shake or wobble. It's pretty obvious that the camera must have hit a part of the set or was bumped by a crew member. (00:02:45)
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.
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: Two white lights reflect off Armus' pool when Picard walks up to it. Since the planet is devoid of all other life, this can only be a reflection of stage lights.
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: 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 walks past the display of the ship the lighting rigs, camera and sound equipment are reflected in the panels.
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.
Hide and Q - S1-E10
Visible crew/equipment: When Riker, Dr. Crusher, and La Forge enter the area where the injured colonists are located, the mic is visible for a few seconds at top left of the shot.
Visible crew/equipment: Numerous times in the bridge scenes, the studio lighting rigs are seen in reflections on the work station screens.
Visible crew/equipment: In the opening scene, just before Picard comes out of his room, you can see the shadow of the boom microphone.
Visible crew/equipment: Boom action is visible in reflections on computer consoles in the back, while Picard and LaForge talk to each other on the bridge. (00:40:15 - 00:40:46)
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.