How do I find the origin of an Instance?

You might be able to use debug.traceback() but it would have to be called from a script that is linked to where the part is created.

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Code from the wiki:-

-- Ran from a Script in workspace called "Script"
local function a()
    print(debug.traceback())
end
function b()
    a()
end
local function c()
    b()
end
c()

Output:-

Script 'Workspace.Script', Line 3 - upvalue a
Script 'Workspace.Script', Line 6 - global b
Script 'Workspace.Script', Line 9 - local c
Script 'Workspace.Script', Line 11
Stack End

Although you can not directly track what script made an instance, you can make a function that acts as a proxy to the instance.new function and track what script is using that proxy. This can be done with a ModuleScript:

return function(class, parent)
    -- The script that called this function is set to getfenv(0).script
    return Instance.new(class, parent) -- You should handle errors made from this function so not every instance creating script breaks (if 1 script uses Instance.new incorrectly).
end

You can then have each script that creates instances use this ModuleScript: Instance.new(class, parent) would become require(ModuleScript)(class, parent).