Someone on the loose has escaped. If a bear gets out of the zoo and is roaming the streets, he’s on the loose. If your friend sneaks out even though she’s grounded, she’s on the loose.
Things that are loose are not tightly controlled. That can help you remember that anyone on the loose is free after being confined. If your dog is on the loose, he’s not on a leash. If a pet bird is on the loose, she’s out of her cage and probably frolicking in the treetops. If a criminal is on the loose, watch out!