It’s a lantern. Lit. Looks carelessly discarded, but thankfully the glass is still intact. The sorts of gases you find in the sewer are pretty similar to the sort you find in swamps, and not exactly safe to expose to flame.
Which means someone probably didn’t toss this for that very purpose – it would be pretty easy to fling with enough force to break it. Why is it here, then? Was someone using it for light, only to drop it in a hurry?
Maybe. Maybe they were headed for the other hole and saw the muckbeast, and panicked. Doesn’t give you much clue as to why they’d be down here in the first place, though. Or where they are now.
For that matter, it’s not as if you can be sure the muckbeast is what scared them. Best to be on guard.
You advance down the passage until you can see the muckbeast’s back feet. It’s time to put this fool plan into action, probably…
Wait. Long said they’d be ready, but you’ll still catch them by surprise. It might work better if you could signal your presence somehow without actually scaring the beast.