NodeJs may be use Windows Socket API ( which include memory leaks , old known bug )
https://connect.microsoft.com/VisualStudio/feedback/details/605621/winsock-c-program-causes-memory-leak
The problem is the WSACleanup will never be called until you shutdown the network services. ( Mixing up ZMq or Nodejs won’t change that fact )
Now, over the time, you will lock more pages of memory and such notice an increasing after the first memory page is full.