Well, you only discover places that you've been to before, so your character must have already been all over Cyrodiil, but no one knows anything about you, you have no reputation and you have no posessions and no house. I think there's a secret group of thieves/assassins which Hides-his-heart is part of. This group seems a lot more deadly than the dark brotherhood or the Mythic Dawn because they do a similar job but remain much more secretive. This group is probably also quite big but spread out. Hides-his-heart took his time to find Francois Motierre so he probably had to travel a long distance over Cyrodiil, which would explain why your character has been all over Cyrodiil, you have to do jobs all around. When you meet Hides-his-heart, he doesn't know who you are, which is why I say that this group must be fairly big. They might even be spread over all of Tamriel.
When the emperor talks to you, your character acts like you've done something actually wrong because you have the option to question the emperor's optimism about you a few times. Therefore it can't have been a petty crime. I think that your character took what Valen Dreth said too seriously and actually thought that you might die in prison but to believe him you'd have to have done something more than a petty crime.
When the emperor talks to you you get to act all surprised that he's chosen to redeem you without even knowing what you did. As he doesn't know what you did, you were perhaps only found doing a petty crime involved with being in Hides-his-heart's group, but in reality you did much more. If the emperor knew you were a serious criminal, he wouldn't have been as nice to you. You also weren't found as a serious criminial because the emperor would know who you are.
Therefore, you were a serious criminal in Hides-his-heart's gang but you weren't found guilty of it and you stopped being part of the gang because you realised that what you were doing was horribly wrong.