I think the correct grammar would be "Didn't you hear the bell? They are warning us to leave now!"
If Cytha is unaware of the reason, and if she automatically connects the ringing of the bell as being a warning of danger stirring, she could perhaps say "Didn't you hear the bell? That signals a warning! I'm unsure what's happening, but come with me, we have to leave urgently!"
I'm not saying my English is any good here, but it's another example.