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.
"Do jo fink ai kaer if uee luhs a badl bruva oh tuh? Dis is uoah! UOAAAH! Ahl uee nihd is enuf badl bruvas on deh graund dat dey kan luht from auah enehmis an kleim foh de cjapta! Dat ees uat madrs, spess mahreen!"
-Indrick Boreale, during the Kauravan conflict against the Orks.