My response is: ? and go back to work. I feel a bit mean if they are exceedingly polite and Start with "Good Morning" I might give them a Hi in that case. Eit

My response is: ? and go back to work. I feel a bit mean if they are exceedingly polite and Start with "Good Morning" I might give them a Hi in that case. Either way I go back to what I was doing until I come to a convenient stopping point, typically between 1 and 5 minutes.