I've searched Google and this sub but can't seem to find this being asked.
The Secret Annexe that Anne Frank lived in during hiding was huge. Did no one question what such a large space was used for?
Here's a diagram of the house for those who don't know (I visited the museum for the first time yesterday):
http://www.scrapbookpages.com/AnneFrank/AnneFrankPhotos/Drawing.jpg
She lived in the back section of the house in the attic, and the two floors below. This is a huge space so surely people questioned what it was used for, the employees of the warehouse for example.
Short answer - yes. Several of the warehouse employees expressed suspicions during the Frank family's time in hiding about the building hiding Jews. It's also possible that a neighbor might have seen the hiding families from one of the windows visible on the back side of the house. The ultimate identity of the betrayer has never been conclusively confirmed.
Regarding your specific question of how such a space could go unnoticed, I would argue that 1) Everyone who was in a position to directly question why the space was not being used actually did know about those in hiding. There were several helpers who were quite aware of the Frank family (and others) in the back portion of the upper floors. This included the office workers (secretaries) and the main businessmen of the company formerly headed by Otto Frank. And 2) everyone else did not have enough information about the full extent of the building to have reasonable suspicion.
Consider this: there are two basic types of people who are likely to have been suspicious and asked questions: the warehouse workers and the neighbors, especially those to the back of the house. The first group likely rarely, if ever, had cause to go into that part of the building where the Franks were hiding, as the warehouse was on the bottom and first floor. The hiding space was adjacent to the offices of the directors and office personnel, who were in on the secret. How often do you go into the office space of the head of your company? Even if a small company? The second group, the neighbors, would not have had cause to wonder why the space was not being used, as they were not employees. I'm sure you've been in businesses before where the entire building was not open to the public. As far as neighbors knew, it was being used for something business related.
Finally, I would point out that the basic premise of your question is likely incorrect. Someone did question the situation enough to call the police and report the building. The Franks and their fellow hiders were caught, after all.
Sources: Anne Frank Stichting
