When they were learning about boggarts in their 3rd year, Lupin says no one knows what a boggart looks like when it is alone. BUT!! In book 5 there is a boggart in a desk. To make sure it’s a boggart Molly Weasley has Alaster “Mad-Eye” Moody use his magically eye to look through the desk without opening it. Doesn’t the mean that Moody would know what it looks like when no one is around it??