Visible crew/equipment: During Data's second dream experience he imagines becoming a bird and flies through the Enterprise and beyond. As he flies past his father Soong standing at the anvil, a crew member is sitting on the floor by the double doors behind him. This has been edited out in some versions. (00:36:10)
Visible crew/equipment: You can see the boom mic above Picards head when he is in his quarters. (00:11:30)
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.
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.