Previous Member in Current Organization

This Condition on the Current Org category tab in Search Builder is used internally for some searches that are built on-the-fly. However, you can use it, too.

It will find everyone who is not currently enrolled in the last organization you viewed, but has been a member of that organization previously. You can also find previous members by looking at the Previous Member tab of each organization, which is the simplest way to find people that were previously enrolled.

You would never want to save a search if it contains a Condition that looks at the current org, because that will change each time you log in.