Cannot Get Rid of a Slide Master Layout

Here's a new one for me: I cannot delete a layout from a Slide Master set. The course has only eight pages, and none of them appear to use the layout.

• I've manually inspected each page to see which layout it's using.
• I put "FIND ME" in 72-point red bold type on the layout to help visually find it.
• I closed and relaunched the file.
• I quit Storyline 2 and relaunched.

No luck--Storyline still says that one slide is using it. Help.