User blog comment:The Milkman/To Build a Guild/@comment-1992639-20120904004511/@comment-5093119-20120904011605

Well, at least the Companions can be random encounters in the wilderness. For example, I was pickpocketing a random Redguard who wanted me to "get out of his face",but then I was caught doing this crime. I received a bounty, and was attacked by Ria out of nowhere, because the Companions do not tolerate crime.

Another time, used Aura Whisper and found Farkas with 2 other companions fighting Trolls out in the Pale. I followed them around for a bit.

However, like you said they cannot be encountered in caves.