A test to show presence of gold.
This is a rather interesting letter and speaks volumes on the company (the caveat applies: if it is genuine).
The incident is a few days old and certainly not yet investigated, but already they are springing to action. Not by asking questions both to the crewmembers and to the organisation behind them, but by increasing pressure on the crews and maybe most shockingly also introducing a non-equal treatment of crew members. The assignment of surprise line checks that only 25% of the crews are subjected to will likely not increase trust and confidence in the group of pilots. Such a policy may be a powerful weapon for the company to weed out people it considers unwanted, seeing that the fail quota will necessarily be higher than with openly rostered line checks.
Or am I misunderstanding this? Does such an unannounced line check come with a different, less harsh failure policy?