Some of the earlier posters answered your question pretty well. Occasionally, I've had sellers who do not want a for sale sign posted in front of their house and do not want the home displayed on the internet. In talking further with a seller like this, they are usually concerned with safety or don't want the neighbors to find out. (which they probably will discover anyway when a Bekins truck shows up in front... ;) )
BTW, I work the Portland market, so if you want, I can probably locate the addresses of the listings you were referring to. I'm happy to email them to you!
Sometime our sellers want the property advertised on the internet but do not want to give out their address on the internet. If that is the case, contact your Realtor to obtain further information for you. Good luck.
After you find the address, use it to also click around PortlandMaps.com - here you can easily see the Permits, Photos, Oil Tanks, near Bus routes, Zoning restrictions, potential Hazards and much more. In this way you can enjoy narrowing your search - before buying gas. I've had Inspections fail to determine serious things. PortlandMaps has recent Tax Info, a better Sales History, and can help you determine what kids of expertise you may need. Professionals and Contractors can walk thru with you on your second look, if desired. Many great properties are passed over for a fairly simple fix item. In reality all properties have SOME issue. Many can be handled before you move in, or help to negotiate on price. Both sites have several kids of photos that can help you see the neighborhood and architecture also...saving you time and gas money. You can email questions to the Agent or Seller directly thru Zillow. Enjoy!