Inadvertently, britney leaves her briefcase at clean 'n dry launderers when she stops to pick up her clothes. the briefcase is

The briefcase is "mislaid property".

Mislaid property refers to a personal property that was purposefully set around its proprietor and afterward overlooked. For instance, a wallet that drops out of somebody's pocket is lost. A wallet coincidentally left on a table in an eatery is misplaced.