As a last resort, you can use a grep-like tool to search within the DCUs themselves: e.g. Using the build in windows findstr command:
findstr /ism "\<idthread\>" *.dcu
ObscureUnit.dcu
IdScheduler.dcu
will list any DCUs with a reference to IdThread.
You may need to check in the library path as well as a unit you only have in compiled format may have a reference.
Of course, IdThread may be used in a unit that is used by another unit that your project uses. You may need to perform this operation recursively.
NOTE: This may not work on a Unicode version of Delphi, as findstr doesn't support Unicode, and I'm not sure if the reference in the dcu uses Unicode. If it does, use a Unicode enabled version of grep, or use the SysInternals Strings
utility to extract the "readable" text from the dcu files.