I have worked at the armed forces and have been granted, signed and photographed for use and handling of secret material within the forces. I am wondering whether it's appropriate to mention I handled such information on my resume, of course without disclosing anything about the context of any confidential or secret information.
I would not write "I worked with secrets" on the CV
If your future job requires certification, background checks, or general ability to work with confidential/secrete materials, then you will be asked whether you satisfy these requirements (or the company will do its due diligence to check)
If that doesn't matter, you will not get better job offer because you write that you worked with secrets. If anything, I would be worried that you want to brag about such access
Put your current clearance level on your resume
The fact that you still have it means you've handled the secrets safely, or at least didn't do something so egregious that you lost it. Getting a clearance is a multi-month process, so already having one can really help a job search.
Don't mention secrets, let the clearance imply that. Be ready to answer basic questions about how to handle data at different clearance levels in an interview.
Yes. It is worth a small supporting clause under a project or job function you list otherwise, provided your prospective employer cares at all about maintaining sensitive information. Some companies have been harmed by mistreatment of sensitive information by employees, so this can be a small factor in your favor. Otherwise it's filler.
For example, if you apply as a project manager to Apple (cares about product secrecy), you could write, "lead this; delivered that; strictly protected confidential information."
No it's just a clearance that comes with a position so that you can do your tasks, I have a couple of them. It's not useful outside of a niche set of jobs and you lose it when you leave the job.
If it's an industrywide clearance of some sort then it would be worth putting in. Otherwise it just looks like you're struggling to find filler material.