2sents, sniffing is just their way of sizing you up and deciding if you are friend or foe. Not being human, they can’t judge you by the location of the creases in your black hat, the length and style of your sheitel, or the exact coverage provided by your snood.
Or (gasp!) the complete absence of any of the above.
The RBSO has provided them with a superior sense of smell, and that’s what dogs use to get acquainted. If you stay calm and let them do their job, they usually won’t bother you.