Trivia: Scotty visits the bridge from "his" Enterprise (from the original 1966 Star Trek series) on the Holodeck. As the original set had long since been destroyed, the captain's chair and center console used in the episode was donated by a fan who had recreated a life size version of the bridge. The turbolift alcove and one of the stations was built on-set (and re-used in the DS9 episode "Trials and Tribbleations"). Everything else was made up of footage from the original series that was looped.
Descent (1) - S6-E26
Trivia: Stephen Hawking guest stars in this episode, playing himself as a holodeck character (The "Poker with Einstein" program). This came about when he visited Paramount Pictures to promote "A Brief History of Time", and then told the Paramount people that he had always wanted to visit the Enterprise. He not only got to visit, but the writers added this special scene just so professor Hawking could appear on screen in a Star Trek episode. Hawking also reportedly stopped by the warp engine, smiled and said "I'm working on that".
Descent (1) - S6-E26
Trivia: The building used in this episode and in part two (S7 Ep1) is called "House of the Book" Brandeis-Bardin Institute Hebrew Campus, in Simi, California. It was also used in the early seasons of the Mighty Morphin Power Rangers as the Rangers Headquarters.
Trivia: Early on in the episode, just after Picard realises what the terrorists are up to. He fights and disables one of the terrorists, a human played by Tim Russ, using what appears to be a Vulcan nerve pinch (although in the script it is referred to as a "carotid artery block"), which he learned from Sarek in an earlier episode. It's ironic that Tim Russ would later go on to play the Vulcan Lt. Tuvok on Voyager, and use that very same move on many an occasion.
Trivia: When Counselor Troi is leading Mark Twain to his quarters, Twain asks if the Enterprise has ever encountered Halley's Comet. It's a well-known fact that Twain was born a couple of weeks after the comet's appearance in 1835.
Descent (1) - S6-E26
Trivia: Admiral Nechayev tells Picard that the Starship Gorkon will be her flagship. In Star Trek VI: The Undiscovered Country, the Chancellor of the High Klingon Council was named Gorkon.
Trivia: In the Holodeck, you see Scotty's right hand open on the glass and later on the bottle. It can be seen that his middle finger is missing. James Doohan kept his hand closed most of the time on screen because he lost it in World War II.
Trivia: Captain Jellico holds the distinction of being the show's only guest star to record a captain's log entry while in command of the Enterprise.
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.