I think that Hook is in a part of the Underworld that Emma can’t easily get to. Even though she is in the Underworld, they are still searching for him. Hook has a gravestone in the cemetery, so maybe that will be how they are able to find him.
Well the underworld does have “parts”. Perhaps Hook is in the Tarturus region.
They already found him several episodes ago, they been filming with hook at least according to reports from fans since epi 513 and there are reports of them filming even with milah on 514 not to mention hook, his brother filming with emma, regina and henry on 515, they are probably at the cemetery looking for a way to go back home, since that appears to be the place where they entered in the UW.
"I offended you with my opinion? Ha, you should hear the ones I keep to myself".