Oren wrote:
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.

That was a question for Larry. I don't think it would either. Can't even
imagine how it could.

I would consider a hairline crack in the lower
portion of the toilet.

A hairline crack might explain the toilet being low, but not the empty
shower trap.

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

Ok...

Came home, took one look at the manhole cover in front of my house and said
"My, it's sure is clean around the seam. I'm guessing someone's had this
cover off very recently."

I going with the "town did something yesterday" theory for now.

No low water situation in the basement bathroom.