The Trade Secret can be anything along these lines -- it does not have to be something that could be patentable if it were novel. It could be something that had once been patented or widely used, but whose existence had fallen out of common knowledge. It could be something that was also known to some competitors, but the user would have no way of checking this.

The only protection users would have for their Trade Secret would be the usual techniques for keeping a secret -- not making the substance of it obvious or describing it publicly, or burying it among a mass of extraneous information ("Information Hiding"). If it involved a process, the process might be broken up into stages so that operating staff would not know details of earlier or later stages in the process. Employed staff might be bound by a contract not to reveal details of techniques observed, during or after their employment period.

Further devious approaches would be possible. A manufacturer might have a genuine trade secret in use to improve their product, they could give it a name (eg the 'Sparklo Process') and describe it, but inaccurately (disinformation), to make their competitors spend effort on a false trail.

A drawback to maintaining something which would be realistically patentable as a trade secret is that someone else might make a successful patent application for the same idea. Further thoughts on this are described here.