Mark and Allen came as the guests of the palace. Then, both of them allegedly went out for a city tour early in the morning but Mark returned alone for the lunch.
Then, Mark left with the princess, and hours later, the princess tells her mother that she finds the real king and warns her to come and meet them in a city that is nearly 150 kilometers away.
And when her mother attends the secret meeting, she sees two mysterious people standing behind the princess and the King. Most importantly, she couldn't sense the cultivation realm of either of them just like with the royal guests.
As long as one isn't a fool, it is easy to link up everything and guess the identities of the masked men. The Queen and the impostor King quickly recognized them.
Mark also knew the possibility of getting found out. The princess had told him that when he wore the mask.
And yet, he still bothered himself to hide his identity because of three reasons.