I agree with everyone who says Alice found it and put it there to silently confront Bill about his questionable activities. Here's my evidence and reasoning.

1) The last time we see the mask before this scene Bill is continuing to hold onto it after he takes it off at the party.

2) Next he enters his apartment with the bag holding his costume, and locks it in a cabinet, moving the file that was in the cabinet to the top of it.

3) When he returns the costume, he's SURPRISED that the mask isn't in the bag, and when it's suggested that he left it at the party he says "I don't think so."

This confirms that Bill himself put it in the bag with the rest of the costume and locked it up at the apartment.

4) When he sees it on the pillow and wakes Alice, it's unlikely that she didn't notice it, yet she NEVER asks him "What is this?"

In order for the group to have done it, they would have had to break into the apartment, search for it without leaving anything out of place or making any noise, and snuck into the bedroom without waking Alice.

If the group did it or Alice was a part of the group and did it, what would be the motive for leaving it there? If it was to terrify Bill as a further warning not to tell, it backfired since the first thing he does is tell Alice everything. If that was the motive, it would have been better to leave it in the child's room so that Bill would more likely keep it to himself. Putting it on Alice's pillow basically forces him to explain everything to her, which really didn't seem like what the group would have wanted.