It's possible that there are files in some other user's trash. The .Trashes folder at the top of each volume has subfolders for each different user, by user ID number (e.g. user 502's trash is in .Trashes/502).
You can see if it for yourself using a command like this (replace VolumeName with your drive name):
ls -la /Volumes/VolumeName/.Trashes/
total 0
d-wx-wx-wt@ 3 _unknown _unknown 102 10 Feb 18:15 .
drwxrwxrwx@ 21 root wheel 782 13 Feb 14:17 ..
drwx------@ 35 _unknown _unknown 1190 13 Feb 14:18 502
Note: you might get a permissions error from this command, either because the .Trashes folder doesn't allow read access (solve this by adding sudo, e.g. sudo ls -ls ..., and entering your admin password when requested); and/or because of the privacy protections in macOS Mojave (10.14) and later (solve this by granting the Terminal access in System Preferences > Security & Privacy pane > Privacy tab > Full Disk Access category, see here for more details).
As you can see, on my USB disk .Trashes folder there's a sub foder called 502, owned by user ID 502 (for reference, my current user ID is 501). Since this user doesn't exist on my system, I see it as _unknown, and my user can't look inside of it, neither delete it. To look inside that folder we need to do it as administrator (i.e. use sudo).
If you are sure that you want to, you can delete everyone's trash by deleting the entire .Trashes folder with a command like:
sudo rm -R /Volumes/volumeName/.Trashes
Warning: as with anything involving sudo ("do as super user", i.e. system administrator) and rm -R, use this carefully. If you type it wrong, it could have ... unpleasant consequences.
Dragging a drive into the trash just means "eject", not "delete contents". Since USB can't be physically ejected, it just unmounts it. Emptying the trash has no effect in this case.
Unplug it, wait a few seconds, plug it back in, and it should show up.
If you don't want to unlpug and plug it back in, see hdiutil attach for CLI solutions, or /Applications/Utilities/Disk Utility.app for GUI.
Best Answer
You could probably write a script with grep and diskutility that opens finder whenever the count of disks changes?