Oh Ammi I've just found these 2 theories, have a look :
Spoiler:
Mr. F killed Jennifer Fairgate
With so little to go on, armchair detectives on Reddit have honed in on the suspicious "Mr. F," a Belgian man who was staying in the room next door the night Fairgate was killed. Per VG, he claimed to have heard nothing that night, and said he learned of Fairgate's death from the front desk when he checked out Saturday morning — but the body wasn't found until Saturday night. He was never interviewed by the police, and he stopped returning Wegner's calls after Wegner began to grill him about Oslo.
"I think Mr. F needs to be investigated," one Reddit user wrote. "Especially considering Mr. F's slip up, it would match that she died on Friday night."
The murderer was in room 2816
Others theorize that Fairgate's murderer was staying in room 2816, as Fairgate had a newspaper in her room that was labeled for 2816, not 2805. There was a thumb print on the bag, which has yet to be identified. Because old hotel registration cards were discarded in an office move in 2010, no one knows who was actually staying in 2816. https://www.bustle.com/entertainment...eries-season-2
Spoiler:
...hmmmmmm, that all could also fit in with your ‘perhaps there was a serial killer’ responsible for other similar unexplained Oslo deaths theory....