After disabling then subsequently re-enabling the global option "Compile ALL Project Folder Sources", I see the following error/warning at the output of every build, even in new projects.

Compiling 'BluetoothRelay' for 'ATmega2560 (Mega 2560) (Arduino Mega)'
Find 'S'files path not found: rc
Find 'c'files path not found: rc
Find 'cpp'files path not found: rc
Program size: 2,220 bytes (used 1% of a 253,952 byte maximum) (1.09 secs)

I also enabled disabled then re-enabled the global options "Show Build Folder" and "Show Build Properties"

Verbose Build Properties output attached

Thanks for the quick response. The message is now gone, but I'm not exactly sure what made it go away.

I verified that the "always include project folder sources" was true, restarted VS, built the project, saw the message again, set the property to false, restarted again, built again, still saw the message, set back to true, restarted and build again. Then the message was gone. 

Not sure what to make of that, but at least the message is gone.

As a side note, I also don't understand why I was getting those messages in the first place- the 'src' directory is in a shared code project that was under the same solution, but I was seeing the message on every project I built, regardless of whether it referenced the shared code project. No worries if you don't have an easy explanation for that, though, I mainly just wanted to remove the info message since it looks like a build warning.

ok, thanks again. Let me also note that I'm running a pre-release build of windows 10 that has BSOD'd twice, so if you can't reproduce the issue, that might have something to do with it.