How to locate hornet nest?
I suggest you look around the perimeter of the house paying special attention to areas where the foundation meets the sill.

Those nasty wasps or hornets will find a way in a small crack or joint of the sill and build nests under the floor joists, especially if you have ceilings in the basement level.  They will also get into the attic under an eave or vent.  During the day you will see them coming in and out if you watch carefully.

I encountered a massive nest in a flip house I was working on. Every morning there were a couple of dozen wasps flying around the kitchen and living room.  They were actually entering the living space around heat pipes in the floor and around wall electrical outlets. (If they were in the attic, they could enter the house through light fixtures.)  I used a stethoscope on the floor to track down the nest, hands and knees crawling across the floor, listening for the "BUZZ". After I isolated the area they were in, it took me days and multiple attacks of grenade type bombs (6 or 8) into the cavity between the ceiling and first floor and several cans (10) of foaming wasp spray shot directly into the outdoor entry point to finally get rid of them.

The piles of dead wasps just outside the entry point got bigger every day.  Thousands of them.  I eventually sprayed insulation foam in the entry point, sealed it and that seemed to end the problem.  Not sure if your infestation can be DIY treated, but if you look around, I bet you can find where they are coming and going in your house.