"Harry Potter, sir, the Malfoy boy is breaking no rules that Dobby can discover, but he is still keen to avoid detection. He has been making regular visits to the seventh floor with a variety of other students, who keep watch for him while he enters —"
--Dobby, reporting in (HBP21)
Late at night, Dobby and Kreacher appear in the common room to tell Harry what they have learned from tailing Draco Malfoy. Kreacher says only good things about Draco, but Dobby says that he has been visiting the Room of Requirment and using what seem to be several different students as lookouts (OP21).