Unknown PC sometimes showing up on sidebar

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Unknown PC sometimes showing up on sidebar
Hi,
I have noticed an unknown pc showing up on sidebar, which means they are in my network. A month ago, the pc name is hpc, and now the name is home-jh. It is pretty annoying they exist there.
I have AirPort Express but I unplugged it for a day and the pc still shows up on the sidebar. I have 4 printers but the printer names do not match hpc and home-jh. I have iPad and iPhone, but I don't think they will show up on the sidebar.
I have run netstat commands on both PC and Mac. Is there any way I can track their public IP or IP since I just know the host name.
I am suspecting a hacker is DNS cache poisoning my router DNS cache because I freqnetly find my Chrome/Firefox/Safari browser usually say it cannot find the website, say, Google, or many big-named sites despite my internet connection is normal. When I wait few seconds or restart my computer, browsers is functioning normal again.
I have googled and searched this Apple forum but all the solutions cannot solve my issue and so I post this thread here. Anybody has any idea that I can track down unknown PC IP?
Thanks.
Repaly
This has nothing to do with Air Drop, that only works on Macs and even then not all Mac. Here is Apple's information about Air Drop compatiblity.
AirDrop with my computer?
Again your systems are secure and they can only see there is a PC out there however if file sharing is turned off then you have nothing to worry about. Again if you want to not be able to see the PC in a Finder Window hover the mouse over the left pane where it says "Shared" then click hide and it will no longer be seen. Another (and easier method) is in Finder - Preferences - Sidebar tab - deselect what you don't want seen under Shared.
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