"He walks as silently as a mouse."
Translation:Hän kävelee hiljaa kuin hiiri.
The hiiri is not the object/target of any action either. This sentence includes a comparison between the subjects of sentences: A mouse walks quietly. He walks as quietly as a mouse (walks).
There are also comparisons where both things are in partitive, for example "Hän juo sitä kuin vettä" He drinks it like (he drinks / others drink) water. because "se/sitä" and "vesi/vettä" is the object.