How do I telegraph to players that, because they let guards escape, the enemy fortress knows they’re coming?

I’m DM’ing for a group of mostly first-time players, and this is my first time DM’ing. At the end of last session they approached their first big dungeon (goblin fortress), attacked some guards, and let them escape inside. They’re now blissfully camped out front of the dungeon. The guards escaped; the fortress is alert; they’re camped out front. Obviously, there should be (fun) consequences for their sloppiness.

I know how to punish the players for letting the guards escape, like getting ambushed in the night, traps in the dungeon, enemies are ready for them, etc. However, how do I telegraph to the players that these things are happening because the enemies know they’re coming?

Since we’re all first inexperienced, I’m worried that anything “special” I throw at them will be seen as “normal goblin dungeon”. For example, I want the goblins to attack them in their sleep, or maybe they enter a room and all the goblins have their bows drawn and pointed at the players. How do I let the players know that’s happening because the guards escaped inside and alerted their buddies, and not just something that normally happens?

Punishing the players for being sloppy sounds like fun. But how do I let them know they’re being punished?