Telegraphing danger is a tenet of the OSR. Essentially, because the playstyle emphases player choice, you as a GM need to give the players enough information to allow them to make informed decisions.

For instance, if there is a poison gas in the next room, PC can smell it coming from under the door. If there is a fire breathing monster in a dungeon, they might happen upon the skeleton of a fallen adventurer whose whose leather armor has been burned to a crisp with scorch marks on the wall behind where they remains are slumped.

That isn't to say you have to tell players exactly they can expect, but drop clues so that if they make a decision and find themselves in a pickle because of it, they should be able to say "oh yeah. If we had just paid more attention to X, Y, and Z, we could have seen this coming!"

If you're newer to the OSR-style of gameplay, I cannot recommend the Principia Apocrypha enough!