Why not ask? If a guy asks a girl (or vice versa) to go somewhere and it’s not abundantly clear it’s not a date (e.g. you’ve done similar things together before, they’re openly gay, or they explicitly said it’s not a date), then it should be assumed to be a date unless clarified otherwise. So if they don’t specify and you’re unsure, then ask.

That said, her leaving is also odd. A misunderstanding shouldn’t be a big deal. Show up the next day and laugh about it, and you’re golden. I wouldn’t be mad if that happened to me, nor should either anon or the girl. It’s just a misunderstanding, it’s really no big deal.