They may be hearing things.

http://www.huffingtonpost.com/2011/0..._n_873861.html

"...hough this particular study size was small, it's worth noting that the findings match up with that of a study published in 2009 in the same journal. That study showed that highly caffeinated people -- those who had consumed the equivalent of three brewed cups of coffee -- were three times more likely to hear or see things that aren't there, LiveScience reported..."

[The latest study seems to suggest that heavy coffee drinkers are more open to suggestion. How to make a true believer: deprive them of protein and hop them up on caffeine.]