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.
Visible crew/equipment: When Dr. Crusher is trying to save the Borg, just before Worf says "lower the force field" a boom mic is visible.
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.
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.