Take French leave
Definition 1 of 1
Pro Tip 1/3Sneaky Exit
Implies a sly or secret departure. Used when someone wants to avoid attention or leave unnoticed. 
Megan took French leave right after the toast.
Pro Tip 2/3Potentially Rude
Can be viewed as impolite because it looks like you don't respect the host or the meeting organizer enough to inform them. 
When Jim took French leave, his colleagues assumed he was upset with the manager.
Pro Tip 3/3Invisible Guest
Used in social situations where a person joins or leaves a gathering without attracting any attention. 
She took French leave right after dinner, and no one realized she was gone.