Translation:I went down the stairs but she wasn't there.
Eh, I wouldn't say poetic. I use it in normal conversation. It is not very common, but certainly not just used in poems. The thing is, people are subconsciously making their vocabulary more efficient; If I can say "I went to the store; I went up the elevator; I went down the stairs", all using a similar structure, why use "traveled", "ascended", and "descended"?
But above all, if it's correct it should be accepted, period. You should not be graded as wrong for something you got right.
Just for fun, I tried the answer, "I went downstairs..." and they accepted it. This fits your Efficiency Theory. it also implies that you were not expecting her to still be on the staircase but rather on the lower level of the building.
My answer 'I went down the steps but she wasn't there' wasn't accepted. 'Steps' is almost a synonym for 'stairs' in English (at least in this context) and should be accepted.