There is a scene in my story where a teenage boy removes a square tile from the ceiling of a room, climbs into the opening, and crawls inside the ceiling. He climbs out through another opening in the ceiling into a different room. (The point is that the door to the second room is locked, and he is trying to sneak in.) I've seen this in TV shows. But what exactly is the removed tile from the opening in the ceiling called? What exactly is the person climbing into? I'm confused as to whether it's called an air duct, a ceiling duct, a ceiling vent, or something else entirely. Does anyone know? It would be really helpful, as I'm stuck trying to explain this in the scene. Thank you.
I'd call it a ceiling panel. I think these are sometimes called "dropped ceilings," as they are sort of secondary ceilings hung below the main structural ceiling. They hide the building infrastructure, like wiring and pipes, and are pretty common in modern office buildings, industrial buildings, colleges and universities and so on.
I just googled this, and there's even a wikipedia entry on dropped ceilings.
https://en.wikipedia.org/wiki/Dropped_ceiling
One thing to be aware of is that dropped ceilings tend to be very flimsy and there's not much holding them up besides a thin aluminum grid. This might not support the weight of an adult, and it would be harder to crawl/climb on than the wooden beams that support the ceilings of older buildings and many residences. I don't know, as I've never tried it. The space within might be tightly packed with ducts and wires too and not have room to crawl through.
The space can be termed a plenum space (if it's an open ventilation return space without enclosed ducts), or a crawl space or access area if it's set up with access doors/hatches to allow maintenance workers into the space as needed. With most modern buildings, a true crawl space wouldn't be necessary, as dropped ceilings in more recent buildings are designed for easy panel removal, so workers can just use a ladder and remove the panels from the area they need to repair, do the work from a ladder, then replace the panels when done. I've seen them doing this at colleges and universities.
Here's a guideline for maintenance workers for a couple of buildings at a local university.
https://www.csueastbay.edu/af/depar...foraboveceilingentrywhenAsbestosispresent.pdf
Whether or not someone locking a door would think of someone using the space above a dropped ceiling to sneak into a secured office might depend on the valuables stored in said room, whether or not their workplace has different levels of security or access to parts of the facility for different workers, or on the sensitivity/security of the work done in the locked room. I'm assuming that buildings with secured areas that only certain workers have access to would have some kind of wall or controlled access up in the crawl space above the dropped ceiling.
It will really depend on the building, of course, and you can probably fudge the design enough to make what you want your character to do plausible. Or, you could have him think of using the space above the dropped ceiling, discover it's not do-able (part of a try/fail sequence), and find another way into the office in question. Or falling through the ceiling panels at an inopportune time could be a great "things get worse" moment.