I visited a college campus to see how the students would react to my horrible drawings of them! Watch their reactions!
How to react when an artist gives you an unsolicited portrait?
Believe it or not, some people just like being out in the world and giving small gifts to total strangers with no ulterior motive other than delighting others.
Your artist probably enjoyed handing you his sketch then himself disappearing, and is imagining your puzzlement and surprise with no small measure of happiness. He may even have chosen you because he thought there was less chance of your interpreting his gift as selfishly motivated.
How to react? It’s up to you… but always remember life is fun