I prefer to think that Sirius was just trying to see if Snape was really thick enough to belive him, when Sirius told him that Remus was going down there. Then, Snape, in a trihumph, ran down, not even stopping to think, "*Hey* this is Sirius we're talking about here. WHy would he tell me something so incriminating against his friend?"
but... perhaps, under the same format of Sirius just trying to see if Snape would belive it, he pretended to be talking to someone else about it, just so that Snape could overhear it.