I wouldn't say anything to them about it. It might be an isolatd incident of her being mad she's left alone AND left inside.

Besides, its THEIR fault for not listening to you about making her an indoor only kitty from the start. She wouldn't be missing the outdoors if she wasn't ever introduced to them and granted free access from the very beginning. Though I'm sure they won't loke to hear that! And I'm sure you've already THOUGHT that!