I showed a home this past weekend where there was no sign. My buyer asked why a sign was not put out, or if the REALTOR just didn't have one. (I laughed at that comment)
There was a time when finding a home was driving through neighborhoods where you preferred to live. Now, with the Internet and everything online, it isn't as important. We don't get as many sign calls as a decade ago.
Sometimes when a home is vacant, a Seller doesn't want a sign in the yard, as it alerts burglars it is vacant and could be broken into. This was the case with the home I was showing. All the windows blinds were open, and lights were on but was vacant. I would bet the agent or homeowner returned to the home at the end of the day, shut lights off and closed the blinds.
In my interview process, I let the homeowner know there will be a sign installed in the yard (or the window if a condo), and let them tell me if they would prefer no sign. Sometimes a homeowner is very private, and doesn't want all the neighbors to see the sign, and know they are moving. You just don't know the reason for NO SIGN.