One of my favorite tools as a DM is to let players discover text that hints at larger lore or world events. It allows players to figure things out without having to be told directly.
In your particular case, I would suggest that the goblin garrison would want to either send out scouts to keep tabs on the movements and activity of the hostile party that has given its presence away. Or send out a raiding party to drive them off or capture them.
While the party is camped out, have them notice some strange cracking in brush a little ways off. If they investigate, they might find a small number (perhaps even just one) scout. Don't tell them it's a scout. Have the scout flee but, in its haste to depart, it forgets its spy glass and satchel. The spyglass indicates that it was watching the party. The satchel can contain orders from Commander Hobbes the Hobgoblin (or whomever) that reads:
Forward sentries were waylaid by a small team of warriors. They managed to escape with their lives. Keep an eye on them and report back to me on their activity.
For flavor, you might also have the satchel contain a bit of paper with writing which details the party's actions and serves to indicate that the scout they drove off or killed was reporting back to the garrison.
Dusk. The armored one needs to shower. I can smell him from here.
Moonrise. Camp is quiet. The magical one is carrying a long wooden staff. It might be worth something once we kill her.
Midnight. They've changed watch. The rogue likes to twirl his dagger while he's keeping sentry. Mine's bigger. Should be easy mark.
Moonset. The priest is a filthy dwarf. Seems to have sharp senses. He's looking in my direction . . .
If the party fails to act on the possibility of a scout spying on them, force the issue and have a force recon team move in to attack instead (I'd give them a surprise round of combat).
Essentially, you're inserting some "noise in the bushes" into the scene. You can use that noise to reveal the presence of the scout or the goblin recon team, depending on how you want to play it or how much your players like combat. If you think "I'm going to have the noise be from a scout" and they don't bite, you can switch the scout out behind the screen and use the team to attack instead. The players will be none the wiser that you've changed things on the fly.
If you do go with the recon team attacking, bear in mind that the point of this encounter would not be to pose a real threat for the party. It's to act as a vehicle that allows the party to intercept the commander's orders thereby revealing the fact that the goblin garrison knows they are there.
Mechanically, this means that you'd pick creatures with an XP budget geared towards easy to medium encounter difficulty at most. If you do give them a surprise round, remember that this increases the difficulty of the encounter above the nominal value.