Why do Markarth guards attack me after I left Calcelmo's lab?

I'm doing the thieves guild quests and I was sent to get a falmer translations book from Calcelmo in Markarth. I killed the spider and got the key from Calcelmo to enter his Dwemer museum. Here everything is fine so far.

Then I enter Calcelmos laboratory from within the Dwemer museum. Over there are Wizard Guards that are hostile and attack me. It is also strange that Lydia, who is supposed to help me, keeps telling me that I should not be there and refuses to obey to my commands.

Since I got this problem already previously and went back to an older save game I wanted to double-check, so I killed one, two three Wizard Guards and then backed out out of the lab and returned to the museum. The Guards there were still friendly.

I then walked through the lab, killing the Wizard guards on my way. I did NOT get any bounty message while doing that. I copied the falmer translation stuff from the stone and returned through the lab to the museum.

Now, after I left the laboratory and re-entered the museum, the Markarth guards are hostile and attack me. Even when I put away my weapons they keep attacking me - I don't have a chance to talk to them anymore.

So what's wrong here? How can I finish that quest without making the Markarth guards hostile?

Solution 1:

Okay, in my fifth run through Calcelmos lab I finally found the reason.

Just in the second room (you enter the lab, go a bit further, turn right and go further) there is one Wizard Guard and, sadly, a normal Markarth City Guard. When it locates you, it places a bounty worth 5 on you (probably because you shouldn't be there).

When you talk to him and try to pay him to strike out the bounty you will be teleported out of the lab. Luckily the bounty is so small that you can convince him it's too much work to cope with you and he just leaves you alone.

After not killing this guard, I could have the bounty removed on any other city guard after leaving the lab and the other city guards won't be hostile.

Solution 2:

While I'm uncertain why the guards are hostile -- I usually just break in -- there is an easier way out. It is possible, I guess, that killing one of the city guards who entered the room with the Falmer transcription set off the guards in the museum, although I don't know exactly what's happened.

Anyway, once you get the transcription, just jump down the waterfall on the Wizard's Balcony (which has a quest marker, so it's the intended exit route).