はずでは？

Hi. What is the function of the では here? Is something omitted after では?
Thank you.

Yes, you can think of it as 「はずでは（ないのか）？」.

The nuance is that the siren makes him suspicious, and 士道 thinks "(A siren again?) But weren't we able (I thought we were able/we should have been able) to seal away the power?"

Or to put it more abstractly, he's expressing that the situation is contrary to his expectations.

では is often seen used like this. (e.g. そういうことでは？＝そういうことではないでしょうか？, etc.)

Thank you. I see.