I took the practical approach and came up with a strategy that could be optimal (I have not tried to prove it).
In the worst case scenario it takes:
I made a drawing to show my strategy. The yellow squares are the rooms the police officer looks into that night, the black squares are rooms in which the thief logically cannot be.
This answer's a little eccentric, but bear with me. (Warning: this answer would fit a lateral-thinking puzzle, but since it is not explicitly a non-lateral-thinking puzzle, just treat this as light-hearted entertainment.)
There is no killer.
This is probably not the minimum, but it is an improvement to the answer from @Sriotchilism O'Zaic:
with the sizes
In 9 parts:
In 8 parts:
In 7 parts:
In 6 parts:
In 5 parts:
I used the $18$ pieces from the linked puzzle as my starting point and