As per the documentation of db.currentOp:

Changed in version 3.0: For some inactive background threads, such as an inactive signalProcessingThread, MongoDB suppresses various empty fields.

It seems that any idle connections (not running any operation) are suppressing client information. For example, In MongoDB 3.0, idle connections to mongo shell are showing up like this:

{
	"desc" : "conn32",
	"threadId" : "0x7fd6dbf04b40",
	"connectionId" : 32,
	"active" : false
}

Can client (ip/hostname) be included in the output?