Theory:

       “I know I’m right, stupid,” I told him. “By the way, Seven, did you see his scars? I couldn’t, it was too dark, but I bet he got them during a shoot- out with the police or something.”

     “Mummy told us quite clearly they were burn scars,” Nishad said firmly.

     “Perhaps the police had to set his house on fire to force him out,” I suggested. Seven looked unsure.
Explanation:
 
Maya stands firm on her speculation about Mr Nath. She is sure that he is a wanted criminal, for she soon starts talking about his scars. It is understood that Mr Nath is said to have scars, and that the siblings have learned about them from somebody else. 

Maya claims that she never got to see the scars as his house was too dark. Nonetheless, she imagines that Mr Nath must have received those wounds during a shoot-out with the police. Here, one can see that Maya’s speculation on the scars is baseless as she had never seen them for herself.
 
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A man with a scar on his arm

Nishad, however, is not happy with Maya's assumptions. He reminds her that they were burn scars rather than gun wounds. Furthermore, to add to the credibility of Nishad's view, it was their mother who had explained it to them. It is later revealed that their mother, being a doctor, was treating Mr Nath's burns.
 
Despite Nishad's explanation (and not to forget the fact that she never really got to confirm the scars for herself), Maya imagines a theory- one that is likely to be childish and implausible. She suggests that the police must have set Mr Nath's house on fire to force him out, and as a result, he got his burn scars.
 
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'The police must have set Mr Nath's house on fire', says Maya 

As expected, Nishad didn't buy her theory. He instead gave her a look that spoke uncertainty. The line- "Seven looked unsure"- could probably mean that he was unsure as to where Maya's speculations and theories were leading.
 
From the above paragraphs, it could be analysed that Maya's assumptions are born largely out of her whims and fancies. The line,  “perhaps the police had to set his house on fire to force him out”, in particular, explains how Maya is capable of imagining an unlikely situation to justify her stand and perspectives.
 
Meanings of difficult words from the paragraphs:
  
Sl. No.
Words
Meanings
1
ScarA mark left on the skin where a wound, burn, or sore has not healed completely
2
Shoot- outA fight in which two people or two groups of people fight each other with guns
3
Burn scarA mark on the skin produced as a result of an injury caused by exposure to heat or flame
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.