You are not necessarily an asshole for not saying something but it was definitely something you should have mentioned. She's probably not mad at you per say, but one of the most terrifying things to women in general is an unexpected man ESPECIALLY when in vulnerable situations because you just never know what someone would do.

So it's not your fault but just, in the future, say something, even if it's just a "hey BTW" text, even if you think the guy is gone, just to be safe.

Also though: why TF is she naked in bed with her door unlocked and ajar? She can obviously do whatever she wants but I don't know if she really has a right to be mad if that's how she goes about handling her her own general safety.