A script to detect movement?

Best way to do this would be a magnitude check from the camera to the players torso, magnitude is fortunately a property of any vector calculation so your script could look something like this

local camera = script.Parent

while true do
    wait()
    for k, v in next, game.Players:GetChildren() do
        local torso = v.Character.Torso
        if (camera.Position - torso.Position).magnitude < 20 then -- assuming 5 studs is a meter.
            soundAlarm(); -- a function you can define for the alarm stuff
        end
    end
end

Hope this helps.

I think you should use FindPartOnRay() for that, but OnTouched() will be fine too.

Main = script.Parent

function onTouched(part)
    --do something over there
end

Main.Touched:connect(onTouched)

Edit

local ray = Ray.new(
        Vector3.new(0, 0, 0), -- Where to start
        Vector3.new(4, 0, 0) -- How long
        )
local part = game.Workspace:FindPartOnRay(ray)
    if part.Name == "Torso" or "LeftLeg" or "RightLeg" or "Head" or "LeftArm" or "RightArm" then
    --Do alarm thing there
end

Here, I wrote a simple script for you to use to do that. Modify the variable, dist, to the amount of studs the player must be within to set off the alarm. Also, make sure you point the alarm variable to the alarm.

alarm = -- point to the alarm

function findNearestTorso(pos)
    local list = game.Workspace:children()
    local torso = nil
    local temp = nil
    local dist = 10
    local human = nil
    local temp2 = nil
    for x = 1, #list do
        temp2 = list[x]
        if (temp2.className == "Model") and (temp2 ~= script.Parent) then
            temp = temp2:findFirstChild("Torso")
            human = temp2:findFirstChild("Humanoid")
            if (temp ~= nil) and (human ~= nil) and (human.Health > 0) then
                if (temp.Position - pos).magnitude < dist then
                    torso = temp
                    dist = (temp.Position - pos).magnitude
                end
            end
        end
    end
    return torso
end

while wait() do
    target = findNearestTorso(alarm.Position)
    if(target ~= nil) then
        -- start alarm
    else
        -- stop alarm
    end
end