He could've found the address following Ellery from the station. Sevigny knows how to dissappear and stalk. I'm still wondering why he showed up at Ellery's doorstep. I'm thinking...
Me too!
If it's not a public address, he should have known he was not going to be received with open arms (although getting arrested does seem kind of harsh...).
So, what was he trying to achieve? he could be snooping for information, or he could just be sweet on Ellery (and, hey, can we blame him?
)