This is the ending of the sentence:
[...] mais qui cache peut-être de lourds secrets.
Why in this sentence is "de" being used?
I further searched up "mais qui cache de" on Reverso Context and it gave a couple of examples with it.
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If you want to use un instead of de in this sentence, you have to move it from plural secrets to singular secret:
mais qui cache peut-être un lourd secret: but that hides a terrible secret
If it is not clear how many secrets there are, you would say:
mais qui cache peut-être de lourds secrets: but that hides terrible secrets
where de has now a partitive function. Note that:
mais qui cache peut-être des lourds secrets
feels at least clumsy. I wouldn't use it.