Student parties at Durmstrang were forbidden (and difficult to orchestrate): thus, rare and roaring. There had been one last night, but Igor knew this only by deductive reasoning.
Fact: he smelled of vodka and had a pounding headache. Conclusion: there had been a party.
Fact: he had woken up in an abandoned second-floor classroom used for storage by the Transfiguration teacher. Conclusion: The party had been in the second-floor dorms, probably seventh years'.
Fact: He was missing his wand, and someone had stolen all but his shoes. Also, there were footsteps coming up the corridor.
Conclusion: He was in trouble.