I have been concerned about this message also on my new Polo and assumed I had accidentally pressed something by mistake. I didn't find the Manual helpful, so have been trying different scenarios for the last while to see what actually happens.
If the car is locked from the outside (deadlocking the car) and someone is left inside, the alarm will go off after a few minutes if the person moves. However, if the button at the bottom of the driver's door is pressed (de-activating the deadlock), then the driver exits and locks car with key, while someone or perhaps a dog is being left in car, then the alarm will not go off if the person moves inside the car. If the person in the car opens their door, the alarm will not go off. However, if the door is opened from the outside, then the alarm will go off.
This enables you to leave a dog, for example, in the car and allows it to move about, but if someone tried to open a door of the car from the outside, the alarm will go off. The Safelock message is probably to remind you to press the de-activation button if you are leaving someone, or a dog, in the car. It has to be de-activated each time as it automatically re-sets to deadlock.