If that's happening, the vent that serves the toilet is blocked.
STEVEN F: I admire your confidence. It's great to be sure. Unfortunately, what you're sure of is incorrect.
A vent pipe, also called a vent stack, is plumbed into the line just upstream of drains to provide a pressure relief - or in this case, a vacuum relief. That way when a large slug of water is generated the fills the pipe and pulls all the air along with it, it doesn't create a suction in the rest of the piping that pulls the water out of the drain traps in other fixtures. A blocked vent would mean that another toilet could suck the water out of the bowl of this one, even though this one, the one in the question, isn't the one that was flushed. Other flows could cause it too, like the laundry draining.
This is the funny way in which plumbing can contribute to roof leaks - traditionally plumbing vent stacks were routed all the way to daylight, often right through the roof of the house. Now there are air admittance valves that allow plumbers to end the stack within the wall behind the sink, just above the waterline of the fixture itself. But plumbing being what it is, there are millions and millions of stacks above the roof, working just fine...until some idiot bird builds a nest in one, or something somehow falls in and gets lodged. It's not rare, it happens enough that people who work in the building maintenance field have it on their list of things to think about when problems like this are described. And valves being what they are, sometimes those air admittance valves stick, either open, which can lead to a phantom complaint of bad smells in a room, or closed, leading to phantom flushes.
It's okay if you don't know what the problem is, but asserting that you DO, while being absolutely certain in addition to wrong and selling that "information" to others as facts, is negligent at best, and borderline fraudulent. You could convince someone to make decisions that are in fact making their situation worse. Nearly as bad is such a situation as we find ourselves in now, where you don't contribute an answer of your own, but take the time out to criticize a good one that has been provided. This is the kind of thing that makes it difficult for people to make smart decisions.
You're standing in the way of people who are trying to get things done. You're doing it deliberately. Take a step back, Steve.