From a Raymond Murphy book on English grammar, I saw the following sentence:
I saw Sue in town yesterday, but she didn't see me.
Why is there no article before "town"?
Is the following incorrect? Why?
I saw Sue in a/the town yesterday.
In town here uses a special sense of town, which refers to whatever is understood to be the local region. Saying someone is in town is akin to saying they are in the neighborhood or in the area; as the LDOCE has it:
where you live [uncountable]: the town or city where you live:
Cam left town about an hour ago, so he should be out at the farm by now.
I'll be out of town for about a week.
Guess who's in town? Jodie's sister!
Do you know of a good place to eat? I'm from out of town (=from a different town).
We're moving to another part of town.
There are other locations— mostly institutions— where dropping the determiner changes the meaning, like being in hospital or out of school; writing from prison or at sea; or going to church or to court.
For urban (and quasi-urban) dwellers "in town" means in their residential environs. Depending on the context, it might mean in the business district. This is opposed to "out of town," which means traveling away from home.