See DRIVERS-267: the C Driver doesn't check the "hidden" field so a hidden secondary is considered a normal secondary available for reads.

This bug manifests rarely: the hidden node must be in the seed list, and the application must be reading with a non-primary read preference while no primary is available. (If the hidden node is not in the seed list it is never discovered. If it is in the seed list but a primary is available, the driver trusts the primary's host list, which omits the hidden member.)