Tidas didn't get a good feeling from his brother's happy expression. An unknown instinct was screaming at him, but he couldn't understand what it was trying to tell him. All he could do was be cautious, and keep an eye on his movements through the Fae spies.
Maevis and Nicolas trusted Marco the least out of all the humans they'd met. While Skye and Tidas were away in the Highlands, he had disappeared too many times on the Brownie spies assigned to him. This let them know that he was not only aware of them, but was actively trying to ditch them, and generally did a good job of it.
Maevis had informed Skye of his known and unknown movements, and she had told Tidas. The search they had done should've easily spotted him outside of Dragonhorn if all he was doing was hunting.. 'He's planning something, i just don't know what..'