There's not enough information here for a definitive answer (no fault on you, if you had that "definitive information" you wouldn't be asking the question :). That said, I'll guess that there is an "orphaned" photo library somewhere on your HD that's fallen into a crack. I say this because of something that happened to me after Apple "upgraded" iPhoto to Photos not too long ago. In my case this happened after I "migrated" user accounts from an older Mac to a newer one. You may recall that iPhoto allowed you to create as many libraries as you wanted, and over time, I lost track of some of them. This is just a guess, but hope it helps.
It seems that Apple may have been a bit ambiguous as to how to handle removal of data stores containing pictures. There doesn't seem to be a single method that works across all of the different apps, devices and storage scenarios. If you've not migrated a user account (or "upgraded" from
Photos to your 2017 Mac, or the "orphaned iPhoto library" theory doesn't hold up, perhaps one of the following will give you some leads to discover which method will work for you.
This thread might be worth a read
And from Apple's support forum: this might be worth reviewing:
Deleted photos and videos are placed in the Recently Deleted album on your device, where they remain for 30 days. After that, items are permanently removed from all your devices set up for iCloud Photo Library.
You can remove items from your devices before the 30-day time period is up. On your iOS device, in the Recently Deleted album, tap Select, tap to select one or more items, tap Delete, then tap Delete [items]. To remove photos on your Mac, see Access your photos and videos everywhere with iCloud Photo Library in Photos Help for Mac.
And from yet another post on the subject:
- Launch the Photos app on your Mac.
- Click the Recently Deleted folder in your sidebar.
- Click on the pictures or videos you want to permanently delete.
- Click on the Delete button at the top right. A prompt will appear on screen: just hit delete, and you're good to go!