“When someone who is not Jewish notices a mezuzah at someone’s home—and they know what it is and what it means—what is the proper gesture of respect? Touch it and kiss the fingers, another gesture of acknowledgement, or just walk by as if it’s not there?”
If someone who is not Jewish notices a mezuzah, he or she is under no obligation to acknowledge that mezuzah with any gesture. The mezuzah is an indication that the inhabitants of the house are Jewish, and therefore acknowledging the mezuzah is simply a Jewish gesture of connection. Anyone who touches the mezuzah and kisses their fingers or in some way demonstrates a connection to the mezuzah is implying a connection to the contents of the mezuzah.