On Tue, 11 Sep 2012 13:23:08 -0700 (PDT), DerbyDad03
wrote:

Maybe the bird didn't get around to building a nest on/in your vent pipe until recently...

Why would a clogged vent cause water to be sucked out of multiple
traps during the time we were at work?

I don't think it would. I would consider a hairline crack in the lower
portion of the toilet.

Let us know if this gremlin continues once you get home for another
inspection of low water.