If you want to track an invoice in the flow regardless of where it is, you can use the Bookmark feature.

Open the invoice you want to follow and click on the button at the top of the invoice window, the star will then be black, and the invoice will be in a list under the Bookmarks button in the upper right of the window.

Click on that button and you will see a list of the invoices you bookmarked and can see where in the flow they are located.

Click on an invoice in the list and the invoice opens in a separate window.

Remove the invoice from the list by clicking on the X to the left of the line.