Rasputino said:
What was the lie that she told? She was taking care to not to lie, as I remember.
The lie she told was probably in Chapter 368, when Tserriednich asked her if he can trust here and she replied with yes. The guardian beast marked her with its tongue on the same spot, where she got the scar on her face this chapter. And her betrayal this chapter triggered the moment of her lie.
If i am correct with this, it means that be beast will only use its ability if it senses, that you are lieing about something, and then acts on it, the moment you act on the lie, like in this chapter.
But all that is just speculation, i could be completly wrong.