Cats sniff you when you return home and everything new/changed because they
use their incredible sense of smell to "sniff" knowledge. Not so much "nosy"
(that too) but to determine where you've been & what has occurred. Notice
their reaction is different when you've been to the park/bush they get
excited or if you've been to a home with pets, they know... They use smell
to keep track of all their human Mom's & Dad's doings... Cats are wise
creatures who must know everything and use smell to determine much. If you
place a catnip seed (miniscule) on the kitchen floor they will find it,
smell? Cats know where everything is once moved they assess it & store it.
Are they "nosy" or fastidious? Outdoor cats are also familiar++  with their
turf --- survival instinct, knowledge is safety for cats --- anything new
must be viewed, sniffed and assessed. Mari!