Volume. Simply refer to average volume and see how much the daily volume is below the average.
My GGT system is showing volume -40 to -50% of the daily average volume:
If I'm not posting, then you can always check www.finviz.com:
The lack of volume of this magnitude should remove all doubt that the large players are enjoying the Christmas break. I think a separate indicator is not needed.
Regards,
pgd