what is the freactio gregor's sister whenever she enters gregor's

Here is a quote from the text,

"she was so shocked that she lost control of herself and slammed the door shut again from outside. But she seemed to regret her behaviour, as she opened the door again straight away and came in on tip-toe as if entering the room of someone seriously ill or even of a stranger."

Gregor would hide under the couch when his sister turned the lock. It was a sign so she didn't have to see him.