beachy
Well, yes and no.

There are cases in which cadaver dog alerts were admitted in which the alerts did not actually lead to the finding of a body.

One was the infamous Laci Peterson murder case in California in the US in which the judge admitted cadaver dog alerts into evidence:

http://findarticles.com/p/articles/mi_q ... _n14582237
(Laci Peterson's body was eventually located on the shore of San Francisco Bay, but not in the area near the pier where the dog alerted, and not for several months after the dogs were used.)

In the case of the 2005 murder of Janine Sutphen in North Carolina USA, a judge allowed into evidence the fact that a cadaver dog had alerted in the home she shared with her husband, Robert Petrick, and to the trunk of his car. Sutphen's body was found in a lake after her car had been found in a parking lot, and her husband's defence team argued at trial that she had been abducted from the parking garage. No other evidence that she had been murdered in the home was presented. Petrick was convicted, and his conviction was upheld on appeal.

http://www.newsobserver.com/news/crime_ ... 54610.html

I have no idea whether the courts in Portugal would admit evidence of a cadaver dog alert if no body has been found, but that would be up to them, and as you can see from the above, it has been done elsewhere.