An employee of the Royal Collection Trust called Marie answered the public's questions on Instagram and one quizzed her on the secret door inside the White Drawing Room.
WATCH: An insider tells royal fans about the Queen's secret door
Marie explained that there is a cabinet which conceals the secret door and that the passageway allows "the Queen to enter the State Apartments".
At first glance, the grand golden mirror and dresser is just like any other in the room, featuring the same candlestick ornaments and decorative jars. But the floor-to-ceiling mirror actually doubles up as a door, and if you look closely you'll see the gap.
Try and spot the secret door when you're on the tour
Behind the mirror is a much smaller door that leads to the rest of the Queen's private apartments. What's more, the ornaments are permanently glued to the dresser surface, so there's no risk of anything falling or breaking when the door is pushed.
Make sure you check out this genius feature if you're ever inside the monarch's grand London home for a tour.
This is not the only one of Her Majesty's homes which has hidden elements, as over at Windsor Castle, which is now the monarch's main residence, there is a secret underground tunnel.
In a BBC documentary, The Queen's Palaces, Fiona Bruce revealed the castle's secret passageway – and the hidden exit is like something out of a James Bond film.
Windsor Castle has a hidden tunnel
As the newsreader showed viewers into a seemingly normal, unassuming room inside the 900-year-old property, she said: "This is an office just tucked away in a corner of Windsor Castle. But look under here."
Under the carpet, the presenter uncovered a wooden trap door leading to a secret tunnel. "As if by magic, just lift these and the medieval castle emerges," she said.
The military would have used this clever passage to trick the enemy and sneak up behind them, but we wonder if the Queen uses it at all today…