If you are a Vampire with Auspex:Heightened Senses(or the equivalent), AND you have Heightened Senses active AND you see the person with arms-crossed MOVING (as in taking a step or moving visibly to the Player) THEN and ONLY THEN you may make a static mental challenge (you risk a trait, they don't) to try to detect them.
In a static challenge, you rock/paper/scissors with them (or with a storyteller). If you tie, you compare your number of traits in the appropriate trait category with The Difficulty of The Static Challenge. In this case, The Difficulty of the Static Challenge happens to equal the mental traits of the person-with-arms-crossed, so it's *almost* like a regular mental challenge except the person-with-arms-crossed risks NO mental traits.