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.