Oldie, but I had the same issue, so I'll post my solution.
For me, the issue occurred with "Smart cache for CSS" enabled, where one of my module css files had a line
@import "back.css";
So, when cached there was no path reference.
I located the suspect css file via linux grep:
grep -iInrw --context=0 "back\.css" */modules --include="*.*"
In my instance, the css file only contained the above line, so I simply modified the calling php file to call back.css directly.