While we were cruising on the Zaandam on the May 7 through May 21 cruise, we were woken up around 12:30 AM by an emergency signal. The worst part about it there was no announcement as to what the signal was for. The crew was confused as to what to do. After a period of time the captain finally came over the speaker saying some passenger had called in they thought they saw a fire near the funnel. After a time the captain came on and said the fire had been a mistaken report. We were sailing in fog and the passenger mistook the exhaust in the fog and lighting as a fire. Now whoever was on the bridge did the right thing by setting off the alarm, however their mistake was that they made no announcement letting anyone, especially the crew, know that there was a report of a fire and where to look for that fire. It was good it was a false report but if there had been a fire the time lapse in looking for it or dealing with it could have been a very bad thing. I am sure someone on the bridge got a little talking to.