Re: Sent Nexus 9 back but forgot to wipe my information
Where did you send it back to? If you sent it back to Google or any big retailer, they will almost certainly do a factory reset on their own, so your information is probably safe. Did you have a screen lock set up? If so, it's even more safe.
As far as I can tell, old devices stay on your Google Play devices list forever. I still have my old Acer A100 and Asus TF300T on the list, both of which I got rid of a while ago. I don't mind that they stay on the list, actually, because it tells you when they were last used--so it can act as an alert if you notice that the device has been active after you got rid of it.