The young man was staying in the couple's apartment and went one day into their bedroom to look for cigaretts and found the secret compartment where the keys were kept.
Kupp mot Christina | Nyheter | Aftonbladet
Well, the 'Aftonbladet' article is a bit more precise - amazingly detailed actually, because they must have had either access to the police files or the Princess' co-operation as they also publish pics of the crime scene, the hidden safe and the hiding place for its keys - rather strange, IMO.
According to the article, the real story goes like this:
The thief came to Sweden as a minor refugee in 2010, and Tord Magnuson considered himself to be his mentor.
The young man used to live part-time with Magnuson and Princess Christina who regarded him as ‘part of the family’, and he discovered the hiding place of the keys to the secret safe holding the jewels in the couple’s bedroom 'by accident'.
On the evening of the robbery in late May, he was house-sitting while Princess Christina and husband had gone out.
It's stated here that the thief sold all the jewelry, valued at 60.000 Euro, for less than 1000 Euro - but threw the tiara, valued at 40.000 Euro, in a Stockholm river. Divers tried to retrieve the tiara without success.
IMHO the story does have its dubious aspects, but with a newspaper like 'Aftonbladet', you never know.