I am only guessing, but I think it might have been a water based varnish, possibly applied over an existing finish.
Wipe the bare parts with a wet rag and see if the colour changes. If not then there is probably another coating under the clear varnish.
I understand why you would not to sand the whole lot back. If you are willing to darker, you might be able to "throw" more coats on the floor, but I would still recommend sanding back to the bare timber if you want a lasting finish (possibly using an oil/wax finish such as OSMO).
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