If an open fails, you will need to open the file in binary mode, for example:
open input(1,isz=1)"filename"
Then use read record(1,siz=size)d$ to read the binary contents (size needs to be a multiple of 256 and a file has a file header, key blocks, inventory blocks, data blocks, etc). Assuming you know what the data looks like, you can then search 'd$'.