Mel asks Frank to grab her something from her locker, trusts him with the code and everything, and when he opens it he finds her extra set of clothes in the back, neatly folded, including a spare pair of panties. They're clean, obviously, but he'll take what he can get so he glances around the locker room to check if the coast is clear, then swipes them and shoves them into his pocket. He carries them with him all day, absentmindedly thumbing the soft cotton material whenever his hand is free. And when he finally gets home to Abby he barely acknowledges her, mumbles something like "long day, need a shower", and makes a beeline for the bathroom so he can jerk off into them. Later that night he hides them under his pillow, gripping onto them with Abby sleeping next to him. He does debate keeping them, but decides he'd rather leave evidence of himself behind than be selfish, so he puts them back in her locker with his cum still soaked into them.
Maybe it takes her a while to find them because she's never needed to change into her spare clothes before, but she starts her period unexpectedly in the middle of shift and when she goes to change into her spare underwear she finds them already dirty. She's desperate and doesn't even remember that Frank knows her locker code so she kinda thinks nothing of it and assumes she'd accidently brought in a dirty pair so she changes into them anyway and has no idea she's wearing his cum all day