The Frank family and the van Daan's are very wary of being discovered and go to great lengths to ensure that their "Secret Annex" remains a secret. Anne recalls many events in Anne Frank: The Diary of a Young Girl when the families take extraordinary measures, keeping silent at certain times, having noise rules and even forbidding coughing - a dose of codeine seems to cure that problem. They keep the curtains drawn and have a bookcase disguising the entrance to their space.

One evening, Mrs van D (as Anne calls her), thinks she hears loud noises coming from the attic and a few nights later, the family hear "ghostly noises." (Wednesday March 10 1943) It turns out to be rats, fortunately and other than Peter getting bitten a while later, the rats subside, mainly due to the presence of the cat.  

However, Anne recalls, in her diary entry of Thursday March 25, that Peter came to tell her father that there had been noises coming from the warehouse and even someone "fiddling with the door." Everyone was on alert but nothing appeared to come of it and it was supposed that maybe the sounds had even come from the building next door. As a cautionary measure, no one made a sound, not even flushing the toilet. 

The main concern is however, the arrangement of the chairs in the office as they are arranged in a circle, around the radio with the radio tuned in to the English service. Obviously, if the police were alerted to that, their secret would be discovered and they, and those responsible for hiding them, would face serious consequences.