Sylvia answered Buddy, "Alright. Shout if you need help Buddy. I just checked the inside and Muriel's gone. I'm gonna look around a little more," She then headed back inside and investigated the interior of the trap door. 

So far it just appears to be a normal cellar door. However, she then spotted the plague doctor mask. As Sylvia looked at the mask, she asked to herself, "Why would Muriel have this?" 

...Was this Muriel's mask to begin with? And why would she need a plague doctor mask to begin with?

She headed back outside to meet up with Buddy to help keep watch. She also wanted to keep watch on the inside as well, just to make sure that her own body and her friends aren't in danger.