That’s just not the way these motion sensors work. They aren’t actually detecting motion. Instead, they are detecting changes in heat as it passes across the lens of the sensor. (That’s what PIR means: passive infrared)
In order to preserve battery life, the motion sensor sleeps most of the time. It wakes up to check if motion is occurring, and if so, it reports that motion was observed. Then it goes back to sleep. It’s not reporting continuously so it can’t track continuous motion as you describe it.
some motion sensors, like the fibaro, allow you to set a very short interval reporting time, maybe even as low as one second, which will eat up batteries very quickly. It’s still not giving you continuous motion, though. It’s just checking more frequently.
The only devices I know which would integrate easily with smart things to give you a continuous motion check are cameras, and I don’t think you want that in the bathroom.
So I suggest you rethink what it is that you will be measuring. Instead of continuous motion, think about two events that might take place at a longer time interval. (While we’re talking about this, I think we may also need to consider which genders will be using the bathroom and their ages. Maybe it doesn’t matter since this is just a fan coming on, but it is something to consider. Or maybe you’re just designing this for a single person household, in which case you could just make it fit that person’s Use patterns.)
OK, back to the original thought. Can you put a second motion sensor somewhat hidden over by the sink so that it will not catch motion until the person is standing in front of the sink? Then you could set up a rule based on the amount of time between the two sensors reporting.