Description
From: http://docs.mongodb.org/manual/reference/command/isMaster/
isMaster.passives
An array of strings in the format of "hostname:port" listing all members of the replica set which have a priority of 0.
isMaster.passive
A boolean value that, when true, indicates that the current instance is hidden. The passive field is only present for hidden members.
isMaster.hidden
A boolean value that, when true, indicates that the current instance is hidden. The hidden field is only present for hidden members.
I believe the description for isMaster.passive is erroneously copy-pasted from isMaster.hidden. I'd expect it to report true if the current node has a priority of 0.
This may be something to confirm with server devs, but I expect hidden notes are inherently passive (can't be elected), but not all passive nodes are hidden. I'm not sure if the isMaster responses actually enforce that logic or if they simply read back from the replica set configuration – although I would expect a node marked hidden but with non-zero priority is probably invalid.