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In my personal opinion, the best way to find and audit every single item on your network is a Seed Discovery for each subnet, restricted to that subnet (using credentials). Seed Discoveries use switch, router and computer MAC Address tables to not miss a single device. If it is connected to the network and uses TCP/IP, a switch/router/computer has to know it is there - that's just how TCP/IP works. Combine that with custom TCP and/or UDP ports and you should then be able to determine the device type as well. But that's just my personal preference 
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