If you copied the Plug Ins and Presets folders from CC2015 (presumably to some other location on your hard drive) then whatever was in there when CC2015 was installed should be in there. If the actions that you used in CC2015 were loaded from a .atn file that lived somewhere else on your hard drive, then it should still be in that location. As noted previously, do a system-wide search for ".atn" as the file extension - you may have forgotten that you kept certain actions in a folder somewhere that has been moved or copied to some other drive.
One of the irksome things about the whole CC upgrade system is that when a new version of an application such as Photoshop comes out (say the upgrade from CC2015.5 to 2017) the default behavior when presented with the "Do you want to upgrade?" dialog is to remove all older versions of the application. Usually, the installer leaves the Plug Ins folder behind in the former application folder for the removed version, but that is about it. In order to keep your older versions, you have to click on a hyperlink looking line of text in the upgrade dialog that says "Advanced Options" - this is where you can uncheck the box that is, by default, checked that says "Remove old versions."
As far as Plug Ins go, as of 2015(.5?), there is a common Plug Ins folder that will obviate the need to copy (or reinstall) plug ins from one version to another. On the Mac, this folder lives at the following location:
/Library/Application\ Support/Adobe/Plug-Ins/CC
That is: [root level] Library > Application Support > Adobe > Plug-Ins> CC
Install (or copy) everything to this folder and all versions of PS CC on your machine will have access to these plug ins - no more need to have multiple copies of plug ins in each individual PS CC version's Plug Ins folder.
Good luck,
kirk