Try the following search command:
/[^ -~<09>]
(you get the <09>
by pressing the tab key). Or if you want to get rid of those nasty tabs, just:
/[^ -~]
That will find and highlight any non-ASCII or control-ASCII character.
If you still have hidden characters out there, you can try this command before the search:
:set enc=latin1
That will prevent any weird Unicode character to show up in your code.