In The Hiding Place, why did Corrie especially worry at mealtime about the possibility of their underground operation being discovered?

The Beje had become "the center of an underground ring that spread now to the farthest corners of Holland".  Dozens of people came by daily with reports and appeals of those needing help, and Corrie feared that "sooner or later (someone was) going to make a mistake", and their operation would be noticed.  She reflects,

"It was mealtimes especially when I worried.  There were so many now for every meal that we had to set the chairs diagonally around the dining room table...the dining room was only five steps above street-level, a tall passerby could see right in the window".

Corrie was afraid that, because so many gatherered at the Beje at mealtime, their situation might look suspicious and their underground network discovered.  Her fears were possibly potentially realized when one day, at the noon meal, when "seventeen of (them) were squeezed around the dining room table", one of the workers noticed that someone was looking in the curtain.  Although no one had requested the service, a man was on a ladder washing the windows, and had a full view of the proceedings.  To allay suspicion, the group spontaneously began to sing "Happy Birthday", to make the large gathering appear innocent.  Corrie went outside to ask the man what he was doing, and he responded that he had been hired to do Kuiper's windows, which was across the street.  Corrie was never sure whether the man was a spy or had simply made an honest mistake (Chapter 8).