Doesn't need to be high pressure to send up particles. Mythbusters did an experiment to see if toothbrushes kept in open air actually collect any fecal matter from the air and they found the toothbrush collecting fecal contamination even far outside of the bathroom. I'm not sure if they did tests with the lid closed.

Every time you flush a toilet, it releases an aerosol spray of tiny tainted water droplets. So if, like many people, you leave your toothbrush in the vicinity of a toilet, does that mean it's regularly bathed in bits of fecal matter? MythBusters Jamie Hyneman and Adam Savage uncovered the dirty truth to this myth by covering a bathroom with 24 toothbrushes, two of which they brushed with each morning — the others they simply rinsed every day for a month.

As experimental controls, the MythBusters kept two untainted toothbrushes in an office far away from the lavatory. At the end of the month-long trial, they sent their toothbrush collection to a microbiologist for bacterial testing.

Astonishingly, all the toothbrushes were speckled with microscopic fecal matter, including the ones that had never seen the inside of a bathroom. The confirmed myth unfortunately proved that there's indeed fecal matter on toothbrushes — and also everywhere else.

It makese sense... for example there's those ultrasonic diffusers out there which send a constant stream of aerosolized water droplets. All it takes to do that is a small pad at the bottom that vibrates at the right frequency (above hearing range in this case, hence ultrasonic). It doesn't take that much and the extent it happens in the average toilet just isn't perceptible.