This might be wrong, but my expirence.

Event creatures spawn outside your area - i.e. outside any workstation area. They then come towards you. I've a big base, surrounded by large earthen walls, meaning no non flying creature can get in.

It does mean they can congregate somewhere outside your walls - might be some little crevice where you find all of them bunched up.

Oddly, it seems they are indefinite as someone said above.