In Dawn of War II many game descriptions of relict that Blood Ravens use use words "given" or "found". Most of these items also once belong to other Space marine Chapters or other organization of Imperium. This and secrecy surrounding chapter started meme about their kleptomaniac tendencies.
Stole it? No that ridiculous. They just found it lying around completely unguarded. Since such sacred and precious piece of technology can't be allowed to fall into hands of Orks or Chaos they simply decided to secure it.
They will give it back when Tech-Marines will make sure it's fully operational after some field tests.