I walked in and the calves all got up!
I was re-reading the paper, "Group Housing and Feeding Systems for Calves - Opportunities and Challenges," written by Bob James and Kayla Machado and presented in March 2013 at the Western Dairy meetings in Reno, NV.
As a result of their study of eleven dairies in Virginia and North Caroline in the summer of 2011 they made this observation about calf behavior:
"When calves are fed twice daily in individual pens, they respond to people entering the barn through increased activity and vocalization. Calves fed via an autofeeder system will not respond to people entering the pen. If a calf does so, it usually means that they have not been trained to the feeder or there is an equipment malfunction." [bold my editing]
When I read this I recalled going into a barn with ad lib acidified feeding stations with the calf manager. When we went into one pen nearly all the calves got up. The care giver immediately said, "Something is wrong." After searching for a few minutes she found that a worker had closed a valve while servicing the feeding equipment and forgotten to open it again to allow the calves to drink.
Much of effective calf management is knowing what is "normal" behavior of calves and recognizing when they are behaving abnormally. This applies to groups of calves as well as individuals. This one of the reasons I encourage managers of group-housed calves to spend time observing their charges - especially the younger ones less that a month old.