When we hooked up I assumed she knew what she was doing so I didn't bother to ask "Are you seeing anyone?" because I would've expected a response like "Would I be hooking up with you if I were?"
seems unorthodox for something casual
Once you reject the orthodox assumption of monogamy, you should be prepared to reject the rest of the cultural relationship/dating orthodoxy. You should not be surprised to learn not everyone shares your assumptions/expectations about how non-monogamous relationships ought to work. This is highly ranked among the reasons you hear polyfolk chant "communicate, communicate, communicate!"