- You can see through the narrow passage a yellow glint of metal and what looks like a door in the wall beyond;
- There is another route to the same room, but it has an obvious danger; the narrow passage is easier;
- It's possible to squeeze through while wearing armor, but you might -- might -- get stuck;
- Removing armor isn't the only option; greasing the outside of your armor improves your chances, too, for example.
Thus, we have a hint of a reward and an obvious obstacle. If we also have an ordinary tunnel in another room that leads in the same general direction as the room with the yellow glint, and if that tunnel is strewn with human bones covered with teeth-marks, we have a hint of danger as well. We have something that the players can choose between.
Of course, the narrow tunnel wasn't quite what I was talking about before, since you are really only taking your armor off and leaving yourself temporarily exposed. Something closer would be a rickety bridge over a subterranean lake, with a raging waterfall pouring on the middle of the bridge. There's a risk that the force of the waterfall will knock adventurers into the lake. Standard rules tell us that anyone wearing armor who falls or jumps into the water must discard their armor, and presumably their backpack, or drown. That represents an actual loss of equipment, although potentially the adventurers could try to recover it. If they remove their armor and backpacks beforehand, they leave themselves vulnerable, but it won't be as difficult to recover their possessions afterwards (barring something like that theft of the spacesuits scene that RCK mentioned in the comments.) Or, other options present themselves. Maybe they can build a raft. Maybe they can build a raft for just their gear and pull it along on a rope.
In all these cases, there's a clear hint of what could happen. That doesn't mean that the rust monster (or the giant termite) are out of the running. Consider the worst-case scenario: the players don't know about rust monsters beforehand and encounter one randomly. The first attack will probably start rusting something; at that point, the players now have information and can make a choice: drop some iron spikes to distract the creature and run, pull the fighters back and send the magic-users to the front line to hit it with sticks. Maybe some other choices as well. Of course, the better option would be for the adventurers to ask around and maybe hear about a brave warrior who fled the dungeon when his sword crumbled to dust.
But then, that, too, is a choice, with a risk.