“So, I heard you're leaving us.”

A co-worker told you that another co-worker is leaving the company and taking a new job somewhere else. You want to talk to the person who's leaving to say goodbye. You start the conversation by saying to her:

So, I heard you're leaving us.

So...

This is a good general conversation starter. It's often used right after engaging in a little small talk, to transition into more serious conversation. One of the purposes of using this word at the beginning is that it announces that you are going to bring up a topic but gives the listener time to prepare before you ask a question.

So, how have you been?

So, let's talk about what we need to do to prepare for this conference.

heard (someone)(is doing something)

This means that someone told you this fact. You use "I heard..." when you don't want to specifically say who told you:

I heard he was let go because there was a personal rivalry between him and the chairman.