How to find hidden folder containing the flag without bruteforcing?

I’m solving the CTF and there is this task:

Your friend asked you to check the security of a simple Public website that hides a flag in one of the folders on the server. He says he stores his most secret secrets in the same place, so he wants to make sure he hasn’t made a mistake somewhere and his files are safe. Is it even possible to go wrong in this case?
Task link:
The flag is in the format FLAG{}. It’s pointless to brute force the flag, the folder name is complex enough. It also works in another way.

Do you have any ideas? I tried Dirbuster or simple search for files (images, styles) on the page, but nothing was found…