Rodent advice

[Witsend]

Hi Kevin. Hoping you can help - a rat entered our house through the bath plug-hole on the first floor. The bathroom was installed about 3-4 months ago. We presume it climbed up the inside of the soil pipe for the toilet and sink and bath drains as there is no other means of access (but we are having a company over with a camera to be sure). My question however is this: the pest controller said that this shouldn't happen because of the 'traps' - what are traps? I imagined a hinged 'flap' that lifted to allow waste to escape but then dropped back in to place providing a barrier to rodents attempting to gain access. We contacted the plumber who installed the bathroom and he says the 'trap' is the the same as the u-bend. Is the trap the same as the u-bend or has a vital piece of plumbing attachment not been installed?