Most of the time, no one will know.  If someone is sensitive they may suspect there is something or sense an unfamiliar energy from the vessel, but they aren’t going to know exactly what is different.  Most spirits won’t make themselves known to outsiders or without their Keeper’s permission.  If you choose there are spells, ours is called the “Dead Spell” which renders a vessel completely “empty” to any outsider who comes into contact; sensitive to energy or not.