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Why secondary accounts are findable at all
Most "hidden" accounts are not hidden. They are unadvertised. Someone makes a second profile so their coworkers cannot see it, then reuses a variation of the same handle because inventing a new identity is work.
That habit is the thread. The account is tucked away from their main follower list, but the string in the URL is still something they chose.
The handle-variation method
Start from the known handle and generate the small set of strings people actually pick: the same handle with a trailing digit or year, with an underscore or dot inserted, with a prefix like "real" or a suffix like "backup", or with the platform name appended.
Check those variants against the platforms that matter for the case in front of you — not all 3,000 at once. Ten plausible variants against eight likely platforms produces far fewer pointless matches than one string against everything.
Cross-platform pivots that do work
People link their own accounts more often than they realise. A website field, a Linktree-style page, a cross-posted photo, the same bio sentence — any of these connects a hidden account to a main one using evidence the person published themselves.
Photos are the strongest pivot. A profile picture reused across two accounts is a much better link than a matching handle, because the handle could be coincidence and the image usually is not.
What a hidden account proves — and what it does not
Finding a second account proves a second account exists under a related handle. It does not prove deception, and it does not prove the owner had a reason to hide it. Plenty of people keep a quiet account for hobbies, family, or fandom.
Judge the conclusion by what you can show, not by what the discovery makes you feel.