If there is a gun on the mantelpiece in the Act 1, then in Act 2, that gun must be fired.
I'm a big believer in Chekhov's Gun. I try not to do anything or introduce any new plot point without first foreshadowing it, no matter how subtly.
This is easy in film. A lazy way to do it is to have some object appear in the background, or perhaps briefly show it on TV. A watcher who's paying attention gets the reference, a watcher who isn't, doesn't.
It's harder in writing. Every single thing that you mention, for at least a moment, is right in front of the reader's eyes. There's no such thing as background text here - everything is in the foreground.
Sometimes, I want to show the reader that there's a gun on the mantelpiece, without saying "Hello! This is a gun!" - because if I do that, the reader will think "hey, the writer wouldn't do that unless that gun is going to be fired." Sometimes I don't want the reader to know that there's going to be a gunshot in Act 2 - but I also don't want my character to pick up a gun that the reader didn't know was there.
In my specific case, MC's house has a basement, and late in the story he gets locked within it by his housemate. The basement door is locked with a coded padlock to which only the housemate knows the code. MC is not, and has never been, allowed to go inside the basement.
I need to foreshadow the existence of the basement (and the padlock) before this scene.
However, before this point in the story, MC and the housemate do not actually meet beyond letters written to each other. I can't image the housemate would explicitly write "Remember, you're not allowed into the basement!", not least because that would be whacking the reader over the head with an obvious Gun. So MC should probably notice the basement of his own observation.
The story is written 1st-person, from MC's perspective, in a train-of-thought fashion - that is, everything he thinks is there on the page. So I can't have him suddenly jumping to think about some random basement door. And I'm trying really, really hard not to bash the reader over the head with obvious foreshadowing.
What should I do here? How do I hide a Chekhov's Gun such that it's obscured under the bedsheet but, upon later reflection by the reader, was obviously a Gun?