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)
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.