Updated June 15th, 2019.
If your Trash is empty, it might fill up when you connect a disk to your Mac. Then, when you eject the disk, the Trash might look empty again, without you asking for it. This can be disconcerting, but not if you understand why it happens.
The reason it happens is each disk has its own invisible Trash folder, right on the disk. If you throw something away from an external disk it’s not really gone. Instead, it sits in an invisible folder, and shows up in the single “Trash” accessed via the Dock (there’s only one “Trash” no matter how many disks you have). You already know that trashed items are not deleted until you empty the Trash, so it makes sense that they are stored somewhere. Well, if you eject the external disk, all of that disk’s trashed items are also ejected, so if the Trash was otherwise empty, without those ejected items to display, it really will be empty then. And if you connect the disk again, the Trash will fill up again, displaying the items that were in that disk’s invisible Trash folder. Sort of weird, but now that you know how it works, not disconcerting.