She doesn't quite look caught out—she anticipated this too—but Ness does purse her lips a little before she can school her features back to impassive logic. This is not her most compelling point, and she knows it.

"And there have been how many other Rifters who disappeared and have never returned? I am not comfortable risking my entire existence on a vanishingly small chance that I'll come back, or go somewhere else. Thedas, at this point, is a known quantity, where I can at least have a reasonable level of certainty I won't vanish from the world entirely from one moment to the next."

This is where she tries to employ some rhetorical subterfuge—her point wasn't logically sound, it's based on her feelings more than reason. She's afraid, so she's acting irrationally; Stephen would pounce on that in an instant if she gave him the opportunity, but if she can subtly redirect his focus, give him another point to respond to...

"I might be murdered in the war, yes; but I can avoid putting myself in situations out of my depth, I can train myself to become more adept at combat and survival. I cannot avoid or lessen the danger posed by my anchor in any way short of removing it."