I have a replica set (primary, secondary, hidden). In MMS, I see that after few days that the hidden member had 5 connections, suddenly the number of connection jumps to around 600, similar to the number of connections that the primary and secondary have. This happened twice. Last time after restarting the hidden member machine the connections count returned to 5.

In my node.js mongo driver log the following line begins appears after the number of connections increased:
"picked server [hidden hostname]".

Here is my configuration:

rs.rs1:PRIMARY
rs.conf()
{
	"_id" : "rs1",
	"version" : 2,
	"members" : [
		{
			"_id" : 0,
			"host" : "[primary hostname]:27017"
		},
		{
			"_id" : 1,
			"host" : "[hidden hostname]:27017",
			"priority" : 0,
			"hidden" : true
		},
		{
			"_id" : 2,
			"host" : "[secondary hostname]:27017",
			"priority" : 0.5
		}
	]
}