Fusion Cores in the Corvega assembly plant?

Solution 1:

Random drops are random.

It sounds like maybe you looked at a list like this or this.

These are just lists of the items that are fixed, and will appear in the same location every time. Things that are out on tables will always be there, for instance. Things that are in containers, however, are randomly generated.

To see this, save before entering a building. Enter the building, open a container, note the contents. Then, load your save, go back in, and open the same container. It should have different contents, most of the time.

So, you probably found some randomly generated fusion cores. They are sometimes in ammo boxes, in groups of four.

Solution 2:

I can verify Timmy Jim's observation in the comments. Furthermore, according to the wikia,

In addition to fixed locations below, fusion cores may also spawn in groups of four in certain ammo boxes across the Commonwealth.

This supports our observations. You most likely found some and didn't notice when you picked them up (especially if you typically hit "Take All").