Every time guards and scientists passed by Hermione and the other disillusioned Order members had to freeze – holding their breath, not moving a muscle until the people had passed by and gone out of earshot. They might not be able to see through the disillusionment well, but they would be able to hear them if they made any noise. And if they were spotted, then they would have to stun and obliviate whoever had discovered them, and that sort of thing took time and was prone to mistakes unless you were skilled at obliviation, and they didn't want to risk damaging innocent Muggles" brains.