As obviously there is not any windows search feature or Excel built-in function doing what you want, the only real way is a vba code (a function returning a path and file name of #th workbook having a string "YourUDFName(" in any of worksheets, or a procedure creating a list of such workbooks in some destination sheet)

The forum name is "Excel Programming", is it?

A couple of remarks:
You say you use 2 different OS and Office. And Excel workbooks are all in your hard disk. Do you have 2 OS installed on same computer, or do you work with 2 different computers? Anyone who is offering some help does need to know this!

Maybe you must spend some of your time to organize things better in future (having hundreds of workbooks to follow is an incentive enough).
a) Create a folder. For every function you'll have a Excel workbook with this function (really you create and edit your function there). Additionally you have a sheet with list of workbooks (and maybe links too), where the function is used;
b) (The approach I'm myself using.) Have a folder on network drive mapped for all users. For every Excel (or other) application, you have a subfolder (this may be leveled subfolders system) for every application. You have full rights for whole folder, all others have read-only rights. Every application is named like ApplicationName_v###.xlsm. The folder has subfolder Archive (and may have a subfolder Dev, but you can keep development elsewhere when you want. In main subfolder, a latest version of application is stored, and other files needed by user like registry editors. Whenever a new version is added into main subfolder, the old version is moved into Archive subfolder. And it is good to have a workbook with list of users of application in subfolder Dev, or where ever you are design the application. Whenever you put a new version of application on main subform, you have to give a message for all affected users to get a new version of application, and users are replacing their old version with new one. Of-course this setup will not work when users are entering their own data into their workbooks (my applications mostly read data from 3rd-party applications and dispaly the recieved information e.g. graghically, or they work as converters - they read data from file in certain format (*txt, *.csv, *.xls, etc. - any format Excel can read) and write the recieved information to file in another format making various calculations between) - then you have to open user's old version, an update all changes manually from your new version workbook