Theory:

     On the Monday following Mamma’s birthday, Seven went alone with her to the clinic at Girgaum as I was spending the evening with a schoolfriend. When they returned, Nishad told me he’d been to see Mr Nath and I felt most annoyed that I hadn’t been there.

     Seven had been quite upset about Mr Nath’s gaunt appearance and was sure that he was starving. He told me that he had knocked loudly on Mr Nath’s door that evening and said, “Open the door quickly, Mr Nath.”

     The man had opened it and asked him, “Lost another marble?”

     He had obviously recognised my brother.

     “No,” said Nishad. He had taken the man’s hand in his own, and thrust a bar of chocolate into it.
Explanation:
 
On a Monday after their mother's birthday, Nishad accompanied his mother to the clinic at Girgaum (also known as Girgaon), an area situated in southern Mumbai. Maya didn't follow them as she decided to spend the evening with her school friend.
 
Later, when Nishad and their mother returned, he told Maya that he had been to Mr Nath's house that evening. She felt annoyed and disappointed for having missed the chance. She wished that she had been with Nishad while the latter had met with Mr Nath.
 
Nishad described to his sister the events that unfolded at Mr Nath's place. That evening, when he went to Mr Nath's house, he knocked on his door loudly and asked him to "open the door quickly".
 
When the man opened the door, he had instantly recognized Nishad from their first encounter. Knowing Nishad as the boy who had come to retrieve the marbles, Mr Nath asked him whether he had come for the same reason.
 
marbles-1659398_1280.jpg
Marbles

Nishad said 'no' and then took Mr Nath's hand and left a bar of chocolate in it.
 
chocolate-2201991_1280.jpg
Nishad offers Mr Nath a bar of chocolate

As Nishad was disturbed by Mr Nath's impoverished state and was convinced that the latter was starving, he decided to feed the poor man.
 
The act reveals both the innocence as well as the generous attitude of Nishad. Unlike Maya, Nishad's desire to meet Mr Nath arouse not out of curiosity but rather out of goodwill.
 
Meanings of difficult words from the paragraphs:
  
Sl. No.
Words
Meanings
1
AnnoyedTo become a little angry or irritated
2
GauntOf a person who looks lean and tired, especially because of suffering, hunger, or age
3
MarbleA small ball of coloured glass or similar material used as a toy
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Expert Detectives:Sharada Dwivedi (pp. 126-132). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.