James and Andrea L. Moriarty of Vienna are convinced that "lock boxes" are hazardous for the security of houses for sale. That's because their house, listed for sale at $79,500, was apparently entered "without visible signs of force" in late November. Mrs. Moriarty said an estimated $1,800 in property, mostly jewelry, was taken.

A lock box is a device used in the real estate business to leave a house key in a small metal box attached to the front door. Then qualified agents who have common keys to all lock boxes can obtain the door key and enter the house and show it to a prospective buyer when the owner is not home.

The Moriartys said they want to warn prospective home sellers to insist on "appointment only" looking, or to get an insurance policy rider to cover the situation.

However, the Northern Virginia Board of Realtors, which administers the system that includes about 13,000 lock boxes and keys (for which qualified agents pay), said that the Moriarty's is the first such complaint this year. But a spokesman added that the problem of lock box security is something that concerns realtors on a national basis, despite many precautions taken for security.

Thomas Stevens, manager of the Shannon & Luchs office that listed the property and now is handling it on an "appointment only" basis, said that this was the first case of burglary ever tied to a lock box in his seven-year experience. "And my father, who works for me as an agent, said he had heard of none in his 18 years in the business," Stevens added.

"The Fairfax County police said there is no evidence that the lock box was used as a means of entry. Even though there is no sign of forced entry, it might have been done through some undetected means. I'm one of those many persons who have entered their own houses, when a key was not at hand, without forcing entry," Stevens said.

The los Angeles Times recently reported that no evidence had been uncovered linking some 30 burglaries of homes for sale in the Woodland Hills and Canoga Park areas with the presence of lock box on the houses. However, there were 30 burglaries in houses that had lock boxes and none of thehouses showed signs of forcible entry.