They tend to look for darker, sheltered spaces, with little "traffic". We've "had" outdoor cats and our garage under the potato shelf was a favourite spot, but others we wouldn't see again until they come over with their babies in tow.
Maybe try adding a medium sized box, on its side, next to a wall and facing another wall, with some rags/blankets inside.
If the room is too open she may still not be comfortable, try adding some more "hiding"/cover spots around and break up the open space.
Like, consider if a dog got in and went for her babies, does she have options for safety around?