Our local water tower is empty during this winter storm.

I heard my water heater "running" even though there were no faucets on in the house - I immediately thought I had a burst pipe.

After finding no leaks, I turned on a faucet, and there was actually suction through the spout.

Because my home is on a hill, my theory is that other citizens running their water actually drained my house lines. Is this possible? Shouldn't there be backflow valves or something to prevent this?

In addition, should I be worried about my pipes freezing now that they appear to be empty?

Do standard "winter prep" rules apply to mostly drained plumbing?