Translation:I found blood on the stairs! Maybe he fell!
Perhaps she's referring to an unstated person. Her child, for instance.
In my online dictionary, מדרגה is 3rd Hebrew word for "steps," which refers to other things in English. "Stairs" has only one Hebrew word: מדרגות. I wonder whether this is the reason Duo decided it is wrong.
I agree with you that "steps" is not wrong.