People don’t generally like it when things seem to appear from nowhere, they find it jarring, so it’s best to lay some foundations that they may or may not pick up on. Some of these hints can stem from setting and don’t need to be verbal, or even conspicuous, a little literary sleight of hand.
A way to discuss the idea of using setting to drop hints might be to discuss it in relation to film which, as a visual art is good at dropping silent hints. Consider the films you have watched and things that have been seen but not become significant until later on, whether it’s swords on the wall that will be used to impale the villain or a photograph on a desk of a long dead love. You don’t necessarily have to make these stand out in your prose, though something like a sword on a wall will probably stand out. Photographs, for instance, are common and plenty of people have them on their desks so as you describe someone’s office you might mention that there is a photograph on the desk. You don’t have to mention anything about the photograph, simply that it’s there. So if later on there was a mention that this character has a lover that died and their photograph is on the desk the reader has a chance to realise the significance of a detail they might’ve noticed but thought nothing of.
Another method is to distract from a significant detail with a detail that appears more significant. Swords on the wall could be one where everyone assumes the villain will be impaled on the swords but perhaps the villain actually hits their head against the mantelpiece below the swords. If you were to say there were swords over the mantelpiece it’s unlikely anyone would assume the mantelpiece was the potential murder weapon rather than the swords. With a little linguist moustache twirl you’ve tricked the reader into thinking they know what’s going to happen and surprised them when it doesn’t.
Although there’s a saying that if a gun appears in the first act it has to be fired by the end this isn’t strictly true. We may have a gun somewhere in the setting but it might be revealed that it’s decommissioned and won’t fire anything. Alternatively it could be a convincing replica and a character who didn’t know about guns wouldn’t be able to tell until they picked it up and tried to fire it.
It’s important to remember when describing a scene from a particular character’s perspective they are limited by their knowledge so it’s acceptable to describe something but not have them know what it is. In this context things can be described so the reader might be able to work it out, or at least so the reveal feels more like the answer to a question than a head scratcher. An example of this might be in Ant-Man where Michael Douglas’ character, Professor Pym, carries around a keychain that looks like a tank and it’s not until later in the film that this seemingly inconsequential detail takes on importance when we discover it’s not a model tank on a keychain but a miniaturised tank on a keychain.
This is largely the point of the tank because no-one is going to look at a key fob and think it’s anything other than a key fob despite the fact we know Pym can miniaturise things and he likes to be prepared. The detail is hidden within the detail so when the need arises for an actual tank people aren’t frustrated by the sudden appearance because they realise it was a tiny piece of knowledge that was missing from their jigsaw to give the fob context rather a mysteriously appearing saviour.