They appear with no warning
When they sense the rain coming (and they are never wrong about it), ants can move an entire colony inside the house overnight, as happened here. I sat down to type out some morning emails, and a thousand little dots erupted from under the keyboard, spreading out in all directions. I flipped it over to find this – it was pretty much one of the creepiest things to wake up to, ever. Previous infestations have sprung up in laundry pilers on the floor and in my inkjet printer – I was printing out sheets with little black ants embedded in the paper for weeks after that (it was actually a cute stationery effect, although no one actually asked me where I bought the paper).