"The lawyer had lost the documents."
Translation:De advocaat had de documenten verloren.
The auxiliary for "verloren" is sort of a tricky one. Often "zijn" and "hebben" are both correct: "Gisteren ben/heb ik mijn sleutels verloren". In certain sentences tho I would lean more to 'hebben'... But I guess that's more of a feeling than rule. Definitely "Ik BEN mijn sleutels kwijt" ('kwijt' is adverb tho not 'voltooid deelwoord' of a verb). And 'Ik ben een beetje verloren' could also mean you're mentally a little lost. (NB: in Flanders they use it differently again).