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: Numerous times throughout the series, the screws that hold on the panel faces on the Conn and Ops positions are visible.
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.