"town" and "city"

[a.linguaphile]

Hi,

I am curious to know why there is this following difference:

1. He is not in town now.

2. He is not in the city now.


Why is "the" needed before "city"?

Thanks

[Anglika]

"in town" is a collocation meaning he is not around at present. He may be away on business or on holiday or just not in the office that day.