How can I make a monster that moves when a player isn't looking at it?

Here's a Local Script to detect, if Descendants of a Model is in, player's view.

In the script it tells you where you put the code for when "monster is being seen" or when "monster is not being seen"

Look below the script to get a short explanation of how it works.

Also, if this helps, then, please accept my answer. It would be appreciated. :)

Script:

-- you can set the Model yourself
-- read the green. it will tell you where to put your code
local monsterCharacterModel = nil
-- defining monsterCharacterModel

local function noObstructs(position,character)
	-- defining function
	local castPoints = {workspace.CurrentCamera.CFrame.Position,position}
	-- defining castPoints
	local ignoreList = {game.Players.LocalPlayer.Character, character}
	-- defining ignoreList
	local obstructs = workspace.CurrentCamera:GetPartsObscuringTarget(castPoints,ignoreList)
	-- defining obstructs
	if obstructs[1] then
		-- obstruct check
		-- "not looking at monster" code here
	elseif not obstructs[1] then
		-- obstruct check
		-- "looking at monster" code here
	end
	-- close if then statement
end
-- close function

local function ViewPointCheck()
	-- defining function
	for i,v in pairs(monsterCharacterModel:GetDescendants()) do
		if v:IsA"Part" or v:IsA"MeshPart" then
			-- is part or meshpart check
			local part,bool = workspace.CurrentCamera:WorldToScreenPoint(v.Position)
			-- defining WorldToScreenPoint
			if bool then
				-- isVisible check
				noObstructs(v.Position,v.Parent)
				-- calling function
			elseif not bool then
				-- isNotVisible check
				-- "not looking at monster" code here
			end
			-- close isVisible check
		end
		-- close is part or meshpart check
	end
	-- close for loop
end
-- close function

game:GetService("RunService").Heartbeat:Connect(ViewPointCheck)

The Local Script uses Heartbeat Event to constantly call the function ViewPointCheck. The Function ViewPointCheck checks if every Part and MeshPart of monsterCharacterModel's Position is Visible within the Camera's View. And if Visible is true then it call the function noObstructs. The Function noObstructs checks if there are any parts blocking two points. Which the two points are the Camera and the Visible Part. If there are obstructs then the part is not physically visible. If there are no obstructs then the part is physically visible.