Can't find source of formula (hidden worksheet)

I have a cell showing this in the formula bar
='Back Up'!L37

I need to locate /find out where it's reading that value from. But there are no pages called 'back up'. I have checked the hidden pages too.

I notice that when pointing to a page you don't normally see the ' ' punctuation marks. So perhaps It is linked to something different?

Appreciate any help. Thanks.