Could the cat door be closed? You might not remember now, but some cat flaps can be locked so animals can’t get in or out.
I know that you disagree, based on your reply to @Hallt64, but it really isn’t unusual for outdoor cats to hunt. Just because some HOs specially mention this behavior doesn’t mean that others don’t experience it. Similarly, just because you’ve sat for 2 cats that didn’t bring prey home, doesn’t negate the fact that it is normal. You were lucky so far. Even based just on your experience, 33% of outside cats hunt, making it fairly common. See this post for more discussion about the topic: Questions on next cats sitting - #16 by Twitcher
We all learn a lot as we do more and more pet sits, and this was a very unfortunate experience. You now have some additional questions to ask a HO before accepting a sit. If you read this forum enough (and the other platforms that discuss pet sits), you will see that seasoned sitters and HOs recommend not assuming or guessing anything, even the obvious. Don’t assume that the house will be clean, that the sitter will not drink your expensive wine, etc, etc. The horror stories are few, but mind-blowing 