The slugs probably fell out of a bird's beak, or there were eggs in the compost when you bought it. Yuck, they are horrible. Reminds me of a holiday home I once stayed in where, every night, slugs crawled under the back door into the kitchen
We had to put a thick line of salt along the back door, that dealt with them
They definitely thrive in the rain. This year I tried a new method of controlling them in the garden. Apparently they hate the taste of copper, so I put copper tape around the vegetable patch. Not a single lettuce ravaged by slugs!! Which was funny, because the lettuces were also part of my slug control plan. I read that if you dot your vegetable patch with little gem lettuces, they slugs will eat those, and leave your precious crops alone! Too bad I don't like Little Gems (this crop is way too bitter)!
Sprinkle salt on the slugs on your balcony!!