nomanriffat

It was in the theme footer file which was probably added by plugin “Above The Fold Optimization”. It is literally annoying when plugins write into theme core files and they don’t even revert changes upon deactivation. The text “PATH_TO_COMBINED_CSS_FILE” itself wasn’t anywhere in that plugin’s repository as well but I guess it might be their API which messed things up.
Thanks for your support.

Can you let me know which other plugin is causing this? Because I am only using Autoptimize and W3 which are related to cache and there is no any such text as “PATH_TO_COMBINED_CSS_FILE” in entire W3 repository
PS: I am using w3 along AO on other sites and never got any issue there

I had same problem right now and just fixed it. Check functions.php file of your current theme and there will be a long malicious code in base64, remove that code. In my case, the code completely overwrote my existing functions.php but fortunately I had the backup.