Are mice dying in the loft

Sprinkle flour or talcum powder on the floor. Mice will leave tracks in the powder, eventually revealing entry points or nests. You don't have to blanket the entire floor, just near the walls/borders of the rooms, and "as needed" until you have the evidence you seek. Keep in mind that their typical range is 12-20 feet from their nest.

Look for mouse droppings.

Use an ultraviolet light to find urine stains.

Use light. If you can make the room dark, look for sunlight shining through cracks from outside. Or, wait until dark and shine a light through possible cracks, either from inside or outside, and have an assistant on the other side alert you to any light shining through.

Watch for drafts. If you're in a cold climate such as Minnesota, wait until it is -20 degrees Fahrenheit, so that you can feel the cold air blasting through the mouse hole. (That's how I found an entry point recently. The cold air was flowing into a basement crevice such that I could see a spider web waving in the air. Turns out there was a crack where an addition had been put on the house. This was confirmed by shining a light as described above.)

Mouse and rat traps work better with time. Speaking to a pest control guy he indicated that the traps originally smell very foreign to mice and rats. Last time I had to trap rats I put baited but unset traps out so the traps would pick up the body oils of the rodents. The next day, after the bait was gone, I put bait on (stale bread covered in peanut butter, really jammed on there) and set them. I caught three rats in one day, using the same traps back to back. Priming them without being set and reusing them apparently increases their catching ability. I can attest that this works with rats: I caught 16 rats in three weeks using the same trap.