Drain Noise/Toilet Losing Bowl Water

[Ridge Runner]

A friend of my asked me what I thought, but I'll defer this one to the experts.

Two story house - dishwasher and washing machine on ground floor. Both the dishwasher and washing machine happened to be running at the same time. Homeowner heard a 'bubbling' sound coming from upstairs bathroom. When she checked source of noise, she noticed the toilet bowl itself was empty, but the toilet tank was still full. She had never noticed this happening before. Both DW and washing machine seemed to be draining normally. Might this be some sort of venting problem?

 

[Smooky]

My guess would also be some kind of venting problem. Could be leaves or something blocking the roof vent.