The correct form of the verb “find” in this tense is “found”. Somebody found them. That’s the correct sentence for simple past tense. To find is to see something someone else cannot; in this case, it is not clear if that is someone or something. All it says is “somebody found them”. “Them” could refer to keys, books, or people that had been missing.
All of this said, it might help to differentiate between the past and present tenses.
“To find” is the present tense; it means the item is being found at that moment. “Found” is the past tense; the item was found last week, for example. If you find something now, you say you found it three days later.