Normally an agent will remove her sign at the close of escrow. If you are the buyer, the house isn't yours until escrow closes. As others have said if you are the seller you can decide whether there is a sign or not. Buyers do watch signs even if there is a pending sale. There is some advantage to you to have it remain if escrow doesn't close.
Sometimes an agent may hire a sign company which is prompt to put it up (they will loose business if they are slow) but is slower to take it down (the buyer is the first to complain and they didn't hire the sign company).
Palo Alto home values are given at http://julianalee.com/palo-alto/palo-alto-statistics.htm
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