Having an MPLS label inserted into a packet doesn’t increase or decrease its security, so traffic not having the label wouldn’t be any more or less secure.
As far as traffic between P3 and PE2, if traffic is destined for PE2, and the default behavior of penultimate hop popping is enabled, then there would be not be label used for traffic between P3 and PE2. Traffic in the opposite direction, however, would be using a label imposed by PE2 before it is passed on to P3.