With those settings, a "line that doesn't adhere to current tabs settings" is basically "a line with spaces as indentation".
You can highlight such lines by searching:
\v − "very magic" mode so we don't have to use too many
\ later (see
^ − anchor to the start of the line.
\t* − match zero or more tab characters.
+ − match one or more space characters.
\t* − match zero or more tab characters again.
[^\s] − match anything except a space character.
Or if you want to highlight the entire line, add
.* to the end to also match the rest of the line:
If you don't want to clobber your search history with this, you can use
:match Error /\v^\t* +\t*[^\s].*/