Wow, I would be so furious also!
Surely they wouldn't use something harmful if they knew Sadie was in there...but then you never know. Some people are just dumb and insensitive about pets. It's also so wrong that they put you at risk by leaving your security alarm off.
Maybe you should tell the manager you want no one in your apartment when you're out. I sure hope Sadie is okay. Please keep us posted.