Mandie and the Silent Catacombs by Lois Gladys Leppard

Mandie, Celia, Mrs. Taft, Senator Morton and Jonathan leave the problems of Paris behind them as they board an express train whose destination is Rome, Italy. Mandie and Celia are excited about seeing the Colosseum, the Sistine Chapel, the Forum, the Pantheon, and the hundreds of fountains in Italy. They've nearly forgotten the vanishing woman who has followed them wherever they've gone. Without warning she reappears!
  Mandie determines she'll find out who the woman is of it's the last thing she does in Europe. But the fact remains that the woman may be dangerous. And when their sightseeing time in the Catacombs turns into a near nightmare with an accompanying burglary, their time in Rome promises more surprises than even Mandie can handle!
  The Catacombs are no place for Mandie and her friends on their own!