By looking at the code of the InstanceGUID method in the %SYS.System class we can see where it's stored:

/// Returns instance GUID.
/// <br>
/// An instance GUID is a 16 byte (128 bit) globally unique identifier, assigned per instance of Cache installation.
ClassMethod InstanceGUID() As %String
{
    s ns="^^"_$zu(12)
    if ('$d(^[ns]SYS("INSTANCEGUID"))) Set ^[ns]SYS("INSTANCEGUID")=$system.Util.CreateGUID()
    Q ^[ns]SYS("INSTANCEGUID")
}

It's in ^SYS("INSTANCEGUID") in the CACHESYS database.

USER>w ##class(%SYS.System).InstanceGUID()
C59DD3E8-8474-4045-B252-1AF1A0D94F3C
USER>w ^|"%SYS"|SYS("INSTANCEGUID")
C59DD3E8-8474-4045-B252-1AF1A0D94F3C
USER>w $zv
Cache for Windows (x86-32) 2016.2.1 (Build 803U) Wed Oct 26 2016 13:33:05 EDT
USER>

However I don't know what might happen if you change it. Best check with InterSystems.

I don't know if this is related, but recently when I've been posting images in new articles, or in answers or comments, I sometimes upload my standard filename (Capture.PNG) but the image that appears in the DC editor window is wrong. It often seems to be one that I've previously uploaded, though not necessarily my most recent previous upload. I delete it in the editor, repeat the upload, and it's usually OK the second time. I recall once having to do two retries. I'm using Firefox 53.0 on Windows 10.

Anyone else seeing this kind of event?

Did you see my answer above? Did you look at using the ExportAllClasses method of %SYSTEM.OBJ to write a file? And then use the Import method from within another namespace to load the contents of that file?

To help you get started, here's me running the first step:

SAMPLES>s sc=$system.OBJ.ExportAllClasses("c:\s\samples-allclasses.xml")
 
Exporting class: Aviation.Aircraft
Exporting class: Aviation.Classification.Utils
Exporting class: Aviation.Crew

...

Also, you may find the "Add new comment" links in Developer Community useful, instead of posting answers that are really questions. For example:

Please also be aware that you can mark one answer to your question as "accepted" by clicking on that checkmark you see in my screenshot above.

The AutheEnabled property is an integer value that is treated as a set of bitflags. Here's one way of decomposing it:

%SYS>set sc=##class(Security.System).Get(,.props)

%SYS>for bit=0:1:31 if $zboolean(props("AutheEnabled"),2**bit,1) write !,"Bit ",bit," is set"
 
Bit 4 is set
Bit 5 is set
Bit 6 is set
Bit 10 is set
%SYS>

Documentation for the $ZBOOLEAN function is here. You can use the OR operation (third argument = 7) to set a specific bit within the existing value. For example, to ensure bit 4 (whose meaning is AutheOS) is set:

set props("AutheEnabled")=$zboolean(props("AutheEnabled"),2**4,7)

If you are satisfied with my answer please click on the checkmark alongside its title.

Your reference to LocalSystem means I assume you're running Cache on Windows. On this platform Cache processes started by Cache (e.g. telnet logins, web application handlers, Studio connections) will run at the OS level as whatever Windows account the Cache service is set to log on as (see Windows service control manager tab as shown below).

If you make your Cache service run as a specific account rather than as Local System, then all of the processes started by Cache will run with those credentials.

I don't know if this will help you.

UPDATE: Starting at 2015.2 (I think), it is important to use the cinstall.exe utility (located in your installation's bin subdirectory) to change which account the service runs as:

cinstall setserviceusername <InstanceName> <username> <password>

If you don't do this but instead only  change the setting in the Log On tab of the service (screenshot above), then in certain circumstances the $ZF() functions may return a -1 failure code. See http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=... Search the docs for "setserviceusername" for more details.