it just may be a seasonal thing, that the ants are comming into your house. Arthropods in bathrooms usually means that they are looking for water, or at least a moisture environment either for themselves or the eggs. The same happens with scorpions and spiders.
Insects are really hard to control. You remove one lot only to create an open niche for more to move in. The pest people (and the are pests) say that they can get rid of ants, termite, spiders, scorpions etc... from homes. In reality the poison the ground, cross their fingers in the hope that they do not return. The best thing to do is to seal off your bathroom so they do not emerge into the open. Ants do not eat "bathrooms"
so you can rest assured that your house is not going to fall down or collapse.
If you can send me a picture of the ants I know ant people
(not really ant people, but people who know about ants) who may be able to help. It does need to be a good picture.
The Cape has been collected for more than 100 years. The arthropods there are well described and well known. I'm sure I can find someone who can identify the ant for you.