Is your customer leaving while you're working? If so, they're probably doing it with a friendly smile and you might be inclined to say, "ok, bye."
STOP. Get in that time machine of yours and set it for 20 minutes into the future.
That system password you need to get them to log in and test something.
That test print you need them to do in order to ensure everything is working.
That alarm code you don't have that needs to be set if you're the last one there.
That next appointment that needs to be set NOW so that you're not left "waiting to hear back"
If they're leaving, say "before you go..." and then think of what you may (or will) need from them. It might include the need to reach them for a question, or scheduling a follow-up, or verifying account information. If you're about to log into a 2-factor account, the second factor device may be walking out of the building.
Get all your "before you go" questions inserted before they leave. Don't fall for the friendly smile and "I trust you can take it from here" compliment.