They can reach out at any stage. One cannot specifically identify whether that is done at IP1 or IP2. The reason is that IP1 and IP2 are terms invented by people on this forum. They aren’t real terms. My attorney has years of experience in immigration and he told me that if they do want to reach out to employers, it can literally happen at any stage. He’s seen clients whose employers have been contacted even if clients’ profile shows “We have not started your background check” (which people refer to as IP1). And he has also seen clients whose employers have been contacted shortly after the background check starts (which people refer to as IP2). There are no clear patterns to this.
Also, the online statuses aren’t always up to date. Sometimes, your background check may have started but the online status doesn’t reflect it for a few weeks/days. Sometimes, the background check starts (the so-called IP2) and stays in that state for months. So don’t read too much into those online statuses.