Mereth's Hairband - Baldur's Gate 2 Side Quest
Mereth's Hairband is a side quest that begins by speaking with Mereth in the Hidden Refuge. The Hidden Refuge zone is part of the Enhanced Edition for Baldur's Gate 2 and can only be accessed after progressing far enough in Neera's Companion Quest. You will also have to recruit Neera into your party to access the Hidden Refuge.
For this quest you're tasked with retrieving Mereth's Hairband which is found in the Bridge District of Athkatla in the same location where you originally met her and Neera. Hold down TAB or whatever button highlights containers and you'll notice you can access a single crate in the area. Inside of this crate is where you will find Mereth's Hairband.
A screenshot of the exact crate I am talking about is shown below for anyone who is still confused.
Once you've obtained Mereth's Hairband all that is left is to return to Mereth and give it to her to complete the quest. Mereth will reward you with a Wooden Horse Necklace.