TS is also NBD unless you are not being honest.

Secret should be NBD. TS is a different level.

so your clearance/background investigation will still be good until the next reinvestigation then it will expire after that.

Your clearance has to be attached to a job/org for it to be active.

My agency will hire people with clearances but not stipulate that in the hiring paperwork (as it adds additional time) and just add the clearance once you are on for a couple of months or when its actually needed.

I'd talk to a security officer at you current job about what "keeping a clearance active" even means.

I didn't think you could keep it active without being moved temporarily onto a project that required it.

If your current employer keeps it active, probably. Don’t you have an FSO you can ask?

I am starting a federal job next week. I currently hold a secret clearance (worked as a contractor for many years) but new government position does not require it. Does it mean it will go inactive and I will essentially loose it?

Current company (gov contractor) keeps my clearance active even if not required by the project.