OK, this was puzzling for a while, after update to build 10586, cannot see anything else in WORKGROUP.
NET VIEW gives error 1231 !!
Discovered that this new Windows build has magically become CASE SENSITIVE to machine names!!
IF you have missing shares or completely disappeared machines on your LAN, make sure their machine names are not mixed-case. If they are mixed case, rename them to ALL CAPS and then reboot.
Problem SOLVED, they will no longer be invisible on your network.
Just posting this for the benefit of anyone else who may have machines that are ping-able, and have good DNS name resolution, but it seems that NETBIOS resolution is 'somehow' broken, even though it is properly enabled. What I discovered today was that somehow the latest Win10 build has become slightly CASE-SENSITIVE - and this breaks the old NETBIOS standard going back to antiquity.
So rename your "invisible" shares or machine names to UPPER-CASE, and they will magically appear on your WORKGROUP again.
Microsoft, feel free to send me my bug-finder's bounty-hunter check :-)