Mac Question: Is someone accessing my computer?

I clicked on my HD archive to look for some photographs in the All Images file, and when I did, I noticed that in addition to the >Devices, >Places, >Displayed dropdowns on the left hand side of the window, there was also one for >Shared. Under Shared, there was intel_black, mike_pc, and one other name that was in the similar to mike_pc. I have never seen that before.

I cleaned out the caches, turned off bluetooth, and changed the name of my computer to something more obscure, and the >Shared dropdown disappeared.

I’ve tried looking on apple.com, and googling it, but am stymied as to how to describe this in searchable terms.

Any clue as to what was going on, and was it a bad thing?