The way I do this is by converting my fmb's into fmt's (binary to text), then I can easily search through them.
My forms environment is on Solaris so after I have made any changes to the forms on my desktop I move them into Solaris and execute a script that compiles the binary and then creates the text file using the frmcmp.sh script. When I am looking for a specific item or plsql code, I can then go to the directory that contains my *fmt files and use the Unix grep command to search for that string.
for Windows you might be able to use
FilterFilesWithUnknownExtensions DWORD value to 1 in the following registry key:
Now you can use standard Windows search.