The first time Nico asked Thalia out could be described as many things, but the word that popped out of Thalia's mouth was "random". Just ask her how it happened and she'll say something along the lines of, "It was completely random."
Never mind that he had been pretty flirty with her for awhile (people flirt all the time, no big deal) or that he had been asking her if she was interested in Green Day's concert that was happening next week (that's what friends talk about, no big deal).
No, Thalia never suspected. It had all been so random.