This has happend to me a couple times. I assume that you mean there is a for sale sign posted in the yard.
This is what I have done in these cases.
1. Document to real estate company's name
2. Document the realtors name
3. Document the realtors phone number
4. Call the realtor to find out what the house is being listed for
5. Document this information on the Property Preservation Inspection Sheet (PPI)
I hope this helps.