Of course, it would be nice to put a drop-down threshold in a hidden door, but this is where physics comes into play. If there is an outflow of air from the room, then there should be an inflow. There will be an influx of air if the interior door is opened and then the hood in the bathroom will work, but as soon as you close the door of the hidden installation and the threshold is lowered - a vacuum will be created due to the tightness of the door leaf against the box and all the steam from the shower, moisture and odors will circulate inside this room, and not go into the exhaust duct. I often give the example of my first order for concealed installation doors. We installed doors in the bathrooms located in the restaurant, and the customer wanted to install a drop-down threshold in this door. Then we had little experience and we set a drop-down threshold :). A few days after the opening of the establishment, the owner of the restaurant called me and asked me to remove the threshold from the door, because due to frequent visits to the bathroom, the door was closed, the hood simply did not work due to the vacuum and all the smells remained, and were not extracted in the process. My recommendation is not to put a drop-down threshold in a concealed installation door, if it is a door to a bathroom or a bathroom.
