User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4902593-20130721060857/@comment-3146930-20130722015736

I always assumed it was because Leo didn't know he was a demigod, so that scent that comes with being a demigod isn't that strong. Combined that with the fact that he was constantly being tossed from foster home to foster home, that scent never stayed in the same place for long so it couldn't build up.

I mean Percy didn't really start running into monsters until the Furies suspected him of stealing the Master Bolt. Grover also says that once you start to realize you are a demigod, the smell becomes stronger. This could have something to do with Smelly Gabe however.