By searching I meant searching at airports. Even if all the terrorist who have been caught are muslims, it does not give you the complete picture. Number of terrorists found is a very small fraction of the number of muslims living/traveling in the country.
I'll give you an example. Assume that a particular fatal contagious disease is found 99% times in males and 1% times in females. But only 0.01% of all the male population has this disease. By just quoting the 99% figure you cannot justify screening the entire population of males alone, because as per your logic, males are 99 times more likely to have the disease than females. Since it is very rare disease (because of 0.01% number), it is a good idea to screen all the population. Plus it it contagious and fatal. OK this is the best I can do to logically explain my opinion.