Some steps is the object of the preposition down.
The preposition phrase down some steps acts as complement to the intransitive verb came, specifying the path followed in coming. Most verbs involving motion take such complements to specify the origin, path and goal of the motion, and these complements readily "stack", as in your example:
I came [down some steps] [into the corridor].
You could add even more:
I came [out the door] [down some steps] [into the corridor] [toward the hall].
It probably looks to you like a transitive verb because at some point in your learning you have been exposed to the rubbishy notion that collocations like fall down are "phrasal verbs"—a term which has been used, and abused, in so many different senses that it has no discernible meaning whatever.