But the mystery investor wouldn't have to disclose anything to...

But the mystery investor wouldn't have to disclose anything to the
market if they were just buying "on-market" up until they acquired
5%. We probably wouldn't even notice them acquiring. We wouldn't
even know that they were a european based investor. It's only
because of the block trade that the market has been informed of their
location. To be honest, it was also a dreadfully kept secret, almost
makes me wonder if they were intending for the identity to be figured
out. The clues were "european based" , "established portfolio of ASX
resources investments", "investments in other ASX sand miners".
that narrows it down considerably.

All IMHO, DYOR