Was I Wrong for Eating My Flatmate’s Food Without Asking?

I got home really late after work and hadn’t eaten all day. There was a takeaway box in the fridge with no name on it, so I assumed it was leftovers nobody wanted and ate it. The next morning my flatmate was annoyed because it was apparently the meal they’d been saving for lunch. I genuinely didn’t know it was theirs, but I also know I probably should’ve asked first.