When U.S. diplomat Frank Strovas and his wife returned home early from a holiday abroad, they discovered some mysterious alterations in the decor of their home in the Yugoslav town of Ljubljana.

In their sitting-room, the floorboards were in the process of being dismantled, their television set was in pieces, and a pair of apparently disconnected legs was sticking out from behind a curtain.

It turned out that the legs belonged to one of five intruders who quickly left the premises on being discovered by Strovas, the director of the U.S. Information Center in Ljubljana.

In a brief scuffle, one of the uninvited guests took a swing at Strovas, slightly grazing his face.

Details of the break-in, which took place last January and was hushed up at the time by U.S. officials here and in Washington, were confirmed Wednesday both by the Yugoslav Foreign Ministry and the U.S. Embassy. An official Yugoslav spokesman said he had no information linking the incident with the installation of secret listening devices at Strovas' home.

There is a widespread assumption among both Western and Eastern diplomats serving here that the Yugoslav secret police is capable of monitoring their conversations both at home and on the telephone.

There have been several burglaries at the homes of U.S. personnel in Belgrade during the last year. Although the Yugoslav police have successfully rounded up those responsible in the other cases, the U.S. Embassy has still not received a formal reply to its inquiries over the Ljubljana incident.

U.S. Ambassador Lawrence Eagleburger has expressed his concern to the Yugoslav Foreign Ministry about the failure to provide adequate protection for the homes of U.S. diplomats. But he is also believed to be anxious that the incident should not be allowed to damage U.S.-Yugoslav relations, which have improved markedly over the last two years.

Yugoslav spokesman Mirko Kalezic said an intensive investigation had been launched into the criminal underworld in Ljubljana, the capital of the northwest Yugoslav republic of Slovenia, but the intruders had not yet been found.

He added that Strovas claimed to have recognized one of the intruders from criminal files, but that the person in question had a strong alibi. The U.S. diplomat is now on home leave, but Kalezic said that on his return he will be invited to go to Ljubljana jail to see if he can recognize a group of alleged burglars who have just been rounded up.