Knowledge is trained only, and you are not trained. Also you grew up in a cave, and know about other creatures almost entirely from your mother's mediocre attempts at describing them.

Also he's in a pretty convincing grass costume.

You've only met one kind of humanoid in your entire life, which coincidentally was also green.

A baby orc.
A baby orc by itself trying to steal his food?
A baby orc means adult orcs somewhere, probably.

Dergaix should just immediately flee.
Or Dergaix could burn it real fast and make the world one dead orc better off.
Call it his good deed for the week. Or maybe just revenge.

You can use Bluff to allow you to use Stealth. A successful Bluff check can give you the momentary diversion you need to attempt a Stealth check while people are aware of you.

"There's something behind you!", Nub shouts.

Also, in case it doesn't work, he'd like to ready an action.

Dergaix doesn't speak Goblin.

Nub doesn't see what that has to do with anything.

Nub has a +3 Class Skill bonus to Bluff (Rogue).
Nub has a -3 Charisma bonus to Bluff (Charisma 4).
Nub has a +2 Skill Ranks bonus to Stealth (He's a level 2 Rogue).
Nub has +2 to Bluff.

A red dragon wyrmling has +10 to sense motive.

Nub is starting to wish he was some other different Goblin, with a completely different array of statistics, perhaps with levels in Bard. It probably wouldn't work anyway, but at least then it would be some other different Goblin about to die instead of him.

1d20 + 2 = 16
1d20 + 10 = 22

Dergaix wins.

Dergaix can tell the baby Orc is bluffing.
Bluffing about what?

He doesn't speak Orc, but probably it tried to lie and say its parents were nearby?
Good enough for Dergaix then.

He should just burn it.

Nub jumps into his Bag of Holding, tying it closed behind him.

...

"You know I know you're in there, right?"

Nub continues not knowing Draconic as he tries to hold his breath.

One week earlier

"Porters need wages. Porters need food and water. Porters need hazard pay.
With a bag of holding, you can go where ever you need.

Get a bag of holding, they said.
It's so convenient, they said.
Sure it's pricey, but it'll be with you forever.
It's an investment, they said.

And then fucking Gorg fucking loses it, and everything we put inside."

"I did not lose it, someone took it."

"It was your watch, wasn't it?"

"Who the fuck would take it? We're in the middle of nowhere. Nothing is here."

"Doesn't one of you have some spell for finding stuff?"

"Sure, I'll prepare Locate Object. Shall we hope whatever took it is still within 520 feet an hour from now?"

"There are tracks."

"What kind?"

"Goblin, I think."

"I'll keep detect magic up, so we can see the bag if we get within 60 feet. That way we just need to catch up to the bastard."

Currently

A band of incredibly frustrated, mildly dehydrated adventurers burst out of the woods.

They have been following Goblin tracks all week. They have killed dozens of Goblins by now. None of them have had their bag. All of them, upon having the situation explained to them, have told compatable tales of a Goblin named Nub who does stuff like that, and where he was last seen. Still it has taken this long.

Nub was wandering what those guys have been up to recently.

Not that he can see them.

Nub is still in the bag holding his breath, actually.

They're not green, so probably not orcs?
But they are bigger, and have weapons. He's not going to fight them.

On the other hand, they're still pretty far away.