Where are functions being called from when errors are vague? I have the following errors which show up randomly when browsing the site. It's hard to reproduce, but also hard to debug. I was able to get it in the error state and have it stay during a reload in order for me to debug it. 

I tried going to the code in question which resides in includes/theme.inc But when going to that line 1525. It doesn't really tell me much. The error states that its missing a tpl file, but doesn't say which one as I'm assuming its due to the $template_file  variable being blank. 

How can I trace where is this function being called in order to find the module that is the culprit of this error?

I don't have xdebug installed as I know that can be used to do the tracing. I also don't have experience with it neither as I've mainly solved most of my php issues just by doing a print_r() 99% of the time and I'm able to find where the issue lies, but this time its different. 

What methods are there to debug it and find the source of the problem?
The Devel module can be extremely helpful in cases like this. 
Once installed, go to the admin/config/development/devel path, and you will see an option for additional error handlers:



If you select the Krumo backtrace option(s), Devel will register itself as the PHP error handler, and it can then show error messages with the full backtrace!