On Tue, 11 Sep 2012 09:56:38 -0700 (PDT), DerbyDad03
wrote:

What could have happened during the day that - vent related - would
have caused the water in the toilet and trap to be sucked out?

High winds? I've seen my toilet water level drop after wind storms,
and it is vented. I don't have a single vent for the entire house, but
several.

High winds is the only thing I could possibly associate with the water
level drop.