I don't understand your co-worker's thought here, Siggy.....
I can smell cigarette smoke, and other types of smoke from a mile away. The odor also sits in a person's hair and clothing.
So why the need to sniff the student's fingers?
Hi, Sunny!
I don't know for sure.

The kids are pretty good kids but if they can slip something past us they certainly will. In a second. And they are pretty creative. I'm surprised no one has done it yet but, if one day he sniffs someones fingers and recoils in horror? I'm gonna bust out laughing. I will be laughing all the way to the employee bus stop!
Ya'll stay cool. Have a great weekend!