There has been some mystery to this. We can't really know for sure, although theoretically it could have been anybody.
One theory is a thief who happened upon the building under the Annexe heard them - when caught, he exchanged information for his freedom. 'Twould make sense.
Another theory, this one perhaps more probable, is that a clerk hired - whose identity is known only by 'M' - found evidence of the hidden Jews and tipped the Gestapo (green police) off. He was known as a 'crafty, disagreeable sneak' by those who worked with him. He may have found a briefcase that Mr. Van Daan (Mr. Van Pels) left while working late at night.
It might have been Miep! It might have been Anne Frank herself!
Well, those aren't probable.
But you can see how there's such a vague idea.
We don't really know for sure, and we probably won't ever, but those are two good theories.
Hope that clears things up!
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Nobody has found out who discovered the Franks... Many speculations have been made, but no one knows "who" betrayed the Franks. Three different criminal investigations after the war failed to find enough evidence against anyone to prosecute. Now, 67 years after the fact, it is doubtful anyone will ever be prosecuted, or even known as the betrayer.
Despite exhaustive investigation into that question, it was never determined who their betrayer was.
Victor Kugler was made to tell them where they were but they also got caught by the noise that they were making.
It is believed that Anne Frank's family was ultimately betrayed by an unidentified informant. The identity of this person remains unknown.