Parts that know when the player is/isn't looking at them?

Using the Camera method is a good idea, do you still have the code with it? If I was in this scenario I would use it. Sorry I cannot use the comments yet, so if you do paste it, ill edit this post again.

EDITED (6:42) - the error you got is because it returns 2 values, a Vector3 and a boolean EDITED (6:44) - Just looked at your code again, the argument for it needs to be a Vector3 also

--[[
It seemed to work for me, but remember it is an example, it is not meant to be good I guess.
]]
local player = game:GetService("Players").LocalPlayer
local workspace = game:GetService("Workspace")
local camera = workspace:FindFirstChild("Camera") --you could do workspace.CurrentCamera
local test = workspace:FindFirstChild("TestPart")
local deb = false 
local counter = 0 

function Looked()
	if (not camera) then return end
	if (not test) then return end
	local a, b = camera:WorldToScreenPoint(test.CFrame.p)
	
	if (b == true) then 
		deb = true 
	end 
	
	if (b == false) then 
		if (deb == true) then 
			counter = counter + 1
			print("Looked af") 
		end 
		deb = false 
	end 
end 

game:GetService("RunService").RenderStepped:connect(function()
	Looked()
	if (counter == 2) then --depending on whether you want them to look at it again before it disappear or disappears after one look depends 
		test:Destroy()
	end
end)

my comment did not complete

Theres one thing wrong with it, you should disconnect the event when (RenderStepped) when the part is no more, and instead of upvaluing the test variable from the main scope, you could pass it as an argument in the function