This is regarding the quest "An Empress' Ransom" in Old Ebonheart. There are two bugs here.
What happened:

1. I stumbled upon the Raathim Ancestral Tomb while exploring the sewers and managed to steal the Death Mask of Empress Katariah.
2. Later on I'm doing the Thieves Guild questline and Wry-Eye Moranie tells me to go find the Mask. I did not have the Mask in my inventory when she told me this.
3. I go get the Mask from my house and return to Moranie.
4. She does not recognize that I have the Mask and tells me to go find it even though I already have it. This is bug #1.
5. I go back to the Tomb and I get a journal entry telling me that I found the Mask.
6. I leave the Tomb and give the Mask to the guards. When the guards took the Mask they also took all my stolen items. This is bug #2.