"She thought that he kept the keys to the house."
Translation:Hun troede at han beholdte nøglerne til huset.
Yes, good eye. The verb "at holde" has two different past-tense forms, the standard holdt, and the more colloquial holdte. The latter is used frequently in the spoken language, and I'd say it's a bit easier to pronounce.
Beholde is derived from holde, but it officially only has the standard past-tense form beholdt. But due to the relation to holde, that informal past form is seeping into the standard speech. It's not correct (yet) to say or write beholdte, but it's likely to become more accepted as time goes on. The same is true for other derivatives of holde, such as indeholde, udholde, or forholde.