The Aperture file photos are all dated prior to those in Photos. Since they have appeared in the old hard drive that I linked to my current iMac I would go there to see them.
Then Photos has created a new library or opened a different, newer Aperture Library instead of opening and migrating your old Aperture Library when you first started to use Photos. At the first launch of Photos it will migrate the Aperture Library or the iPhoto Library to Photos that has last been opened, but only, if it is on a compatible volume, that is currently mounted. Otherwise it will just start with a new, empty library.
You may want to copy this Aperture Library to your new Mac and drag it onto the Photos Library to migrate it to Photos too (if you have enough free storage on your new Mac to hold the copy of the Aperture library). Photos cannot merge libraries, however. If you want to merge all your aperture Libraries into one library, you have to do it in Aperture, before you migrate the libraries to Photos.