Theory:

     Initially, Payal would shut the cage door every morning when she popped in to say hello to Owlie before going to school. She would always find Owlie on her perch inside the cage, though the cage door was still open. But Payal knew Owlie was flying because she found bits of meat all over the room. After some time, Payal stopped shutting the cage door even during the day. After all, people seldom went into that room all day, and Owlie was always asleep, so there seemed little point.

     Every day when Payal came back from school, the first thing she did was to peep into the library. Usually, she would find Owlie fast asleep on her perch. But today, it was different. The cage was empty and there was no sign of Owlie! And the door to the garden was open. ‘Mom!’ Payal howled. ‘Who left the garden door open? How will we find Owlie now? ‘Don’t panic,’ said Mom. ‘She must be somewhere around.’ They shut the garden door and looked in every corner of the room. Behind doors, on top of the tall bookshelves, in every nook and cranny. No Owlie.
Explanation:
 
Every day morning, Payal would go to the library to say hello to Owlie, and she closed the cage door when she left to school. She would always see Owlie on her perch, the rod in the cage, even though the cage door was open. So she stopped closing the cage door during the day. Nobody went into the room during the day, and Owlie was also asleep most of the day. So there was no point in closing the cage door anyway. Payal found that Owlie had been flying during the night because she saw bits of meat all over the room. Her idea had worked; she put the meat on the cage so that Owlie can learn to search for her food.
 
In the evening, after she came back from school, she looked into the library first. Generally, she would find Owlie sleeping on the perch. But that day, the cage was empty. The door to the garden was also open. Owlie was nowhere in sight. Payal cried loudly and asked her mom, who had left the garden door open. She wailed how they would find Owlie again. Her mom asked her not to panic and assured that she must be somewhere around. They closed the garden door and searched every nook and corner of the room - behind doors, on top of the tall bookshelves. But Owlie was not there.
  
Meanings of difficult words:
 
Words
Meanings
shutclose
seldomrare
howledweep and cry out loudly
Reference:
State Council of Educational Research and Training (2019). Term-1 English Standard-6. Owlie - Vijaya Ghose(pp. 101-104). Published by the Tamil Nadu Textbook and Educational Services Corporation.