Then at least the two of the blue dots should be moving. There are actually many variables against Tom being the third blue dot. Edwyrd is having dinner with Lenamare. He then was wondering around until he found the storeroom. Then he was exploring the palace with Gastrope. Exador is also seen rushing alone to his hidden room after being intruded by Rupert. Just check the events around those times. The likeliness of Tom and two of the three demons being close to each other in the evening for sometime is just very unlikely.
There is also the possibily that Tom is level three and would just show as a yellow dot. There could also be a possibility of the mirror having a glitch when it view new demons. There is also the possibility that Tom is a level 6, which is purple or indigo. Such color is easy to miss around hundreds of red and oranges because its hue is close to red and oranges and our eyes isn’t really sensitive to that color. Talarius could just have missed it especially when his attention is focused on the green and blue dots.