Anyhoo -- I certainly don't want to sidetrack the discussion, but the finger on the forehead thing certainly is intriguing.
Perhaps it's to make sure that you get oil from your skin onto your finger to get a better print (after all, handling your bags and documentation while waiting in line would rub the oil off of your fingertips). So, if that is the case, I would imagine that they would wipe off the reader after each person uses it so that the residual oil left on the pad from the previous user(s) no longer remains to 'contaminate' the print you are leaving. To clean off the pad, they would probably have to use something with an alcohol solution to remove that residual oil and that would remove most bacteria and/or viruses. More of a chance to get sick from the person in front of you sneezing or coughing than transmitted via the pad.
Kind of shooting from the hip with that, so would appreciate any correction/confirmation.
With that, please continue with the prior discussion...