I pad is gone

It would then appear that the thief knew your iPad’s passcode.

Get started on the following checklists:

If you think your Apple ID has been compromised - Apple Support

See who has access to your iPhone or iPad – Apple Support (AU)

Start here:

Locate a device in Find My on iPad - Apple Support

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPad has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad have been lost.

If the device is not listed in the “dropdown” FindMy device list as you have indicated, the thief has deleted it from your account …

… and you’ll NEVER be able to use FindMy to locate it.

“Last Location” only applies when the battery reaches “critical low” … and then ONLY for a device still “on” your account.

Apple does NOT constantly monitor your device location so as to allow reconstruction of your past movements.