Sentence topic vs. discourse topic

A distinction is often made made between the sentence topic, i.e. what a sentence is about, and a discourse topic, i.e. what a discourse is about.

Examples

Suppose we are talking about Mike's house:

Mike's house was so comfortable and warm! He really didn't want to leave, but he couldn't afford the rent, you know. And it had such a nice garden in the back!

In the example, the discourse topic is established in the first sentence: it is Mike's house. In the following sentence, a new "local" topic is established on the sentence level: he (Mike). But the discourse topic is still Mike's house, which is why the last comment does not seem out of place.