Mystery Stains

I've spent quite a bit of time cleaning wooden floors this last week. I started by washing with half a bucket of warm water with a drop of dishwashing detergent in it, using a well wrung out microfibre mop.

A zinc strip repair, presumably
hiding some merde

Then I discovered Starwax Nettoyant Parquets Stratifiés Vitrifiés. Like all the Starwax products, it's excellent – effective, easy to use and specifically formulated for laminate or wooden floors, varnished or bare, with out being unecessarily abrasive, acidic or caustic like many home maintenance products are (especially the quick fix, no elbow grease required variety). It's still important not to get the floor too wet and not to use more than the recommended quantity of product, even so.

The Mystery Stains, which appear and disappear,
always along the joints

I've just washed, not sanded, at this stage, so we can move into the rooms temporarily, and because the floors are old, there are still some mystery stains. We've worked out what they are, we think, but can you?

Susan