Twice now I have looked for fusion cores at locations suggested by the fallout 4 wiki guide and they are not there. I'm on level 26. Once at the Weston water treatment plant supposed to be in the easy hut (another gamer said they found 4 in ammo cases inside the main building after lift). And once at a Poseidon power station plant with rad scorpions thst attack once you leave. What would be the reason for this?

User submittals aren't always accurate. The official strategy guide doesn't have any fusion cores listed at that location.

Power core spawns in containers are random, I believe they mostly spawn in army style foot lockers.

The Wiki's are community and fan generated content. While the information is usually correct and good, sometimes things are inaccurate, sometimes things are changed in the game since it was submitted.

If you find a mistake, edit the Wiki page with the correct information or send a note to the folks who manage the site and ask them to correct it.

I'm sure someone found fusion cores in those boxes.

For instance at Fort Hagen there is an outside area where multiple barracks beds can be found, each bed has a footlocker.

On one occasion when i visited each footlocker contained 2 fusion cores each.

On another occasion there were clothes and other things in the footlockers.

On top of gunners plaza there is a locked room also with beds and footlockers.

I visit quite often to loot the gunners and sometimes the footlockers contain fusion cores but more often they contain other items.

So I would say if you visit those two spots there is a chance of finding fusion cores but it's not guaranteed.

Realistically the more containers you look in the better chance you have of finding fusion cores.

For instance occasionally I find them in trash cans.

But if you kill a sentry bot you should be able to loot two cores.