It's a well-known fact that absolutely every single modern model of "WTMD"s can "randomly" select a set percentage of persons walking through, either simulating an alarm, or discretely show a light-warning to the person operating it.
You can read this in any product sheet or user's manual for any of the portals.

Now, you have to realize that random isn't random - it's random like your iPod. If it was truly random, you'd risk listening to the same song 1000 times in a row. Which would be unfortunate. So the machine rarely selects several people in a row (but might) and if it does, then no more than a few.

I'm surprised that people think these things haven't evolved enough to include this function, and that there's any discussion about it.

-SB-