How can I locate an unknown error in my theme files?

Hello,

As you can see from the attached image there is an error somewhere on my theme (took from the Debut theme) which is not showing me where the error is.
If I make an error on purpose in some of my files, the error message says exactly where the error is, but otherwise it shows nothing like you see.
I have linter and the theme CLI is not showing anything to have a problem, however the error still exists.

How can I see where the error is so I can fix that?

Thanks

p.s. This message disappears when I switch to customize other pages of the theme.