Keitaro sneezes in one of the early episodes, and claims that it's his allergy to dirt. Every other time he sneezes, it's in the presence of an Orphenoch - and it seems that's what he's allergic to. Now, when he sneezed for that first time, the only people nearby were Mari and Takumi ....who is later revealed to be an Orphenoch.
Early on in the series, Takumi does not seem to be phased, or surprised by the existence of Orphenochs. At first it seems it's just because of his "I don't care" attitude but later it makes more sense - since he is an Orphenoch himself, it's only natural that he was not surprised.