I suppose that what is meant with the bank documents is that there has been some recent incidents where the bank issueing the guarntee does not exist or know littele about the guarntees but i woud be surpiised of Piensa would do this
Your lawyer should be checkinf this and it is a simple check fir the lawyer to contact the isueing bank
Iwould have liked to have seen a more complete response from Spanishsolicitor as if thes bank guarntees are, as put unlawfull or false, this is of course fraud and they can be imediately reported to the Police and OMIC