They'll just see a dark screen with the Magnetic Privacy Screen for MacBook Pro 13-inch (2016) and MacBook Pro 13-inch (2017). Since it adheres to your display with magnets, there's no residue left on your display, and it's easy to attach and remove as you see fit.
Targus ASM133MBP6GL Magnetic Privacy Screen for MacBook Pro
Apple MacBook Air MYE52LL/A (Early 2020) 13.3 Laptop Computer (Refurbished) - Space Gray; Intel Core i3 10th Gen - Micro Center
Apple MacBook Pro MUW63LL/A (Late 2023) 16.2 Laptop Computer - Space Black; Apple M3 Max 16-Core CPU; 48GB Unified - Micro Center
Apple MacBook Pro MK193LL/A (Late 2021) 16.2 Laptop Computer (Factory Refurbished) - Space Gray; Apple M1 Pro 10-Core - Micro Center
Techprotectus Rugged Hardshell Case for Apple 13 MacBook Air,M2 Chip 2022,Black
iClover MacBook Air 13.6 inch Case 2022 Release A2168 M2 Chip, Plastic Hard Snap on Shell Case & Keyboard Cover & Screen Protector for Newest Macbook Air 13 inch Touch ID, Black
Apple MacBook Pro MKGR3LL/A (Late 2021) 14.2 Laptop Computer (Factory Refurbished) - Silver; Apple M1 Pro 8-Core Chip; - Micro Center
Techprotectus Hard Shell Case with Keyboard Cover for Apple MacBook Air 13.6 M2 2022-Crystal Clear - Micro Center
Techprotectus Colorlife Hard-Shell Case & Keyboard Cover for MacBook Air 15 - Micro Center