ユニクロ was the first word I thought of that was clearly in カタカナ that couldn't be categorized as a loanword. It's the name of the Japanese clothing store Uniqlo, which is essentially the Japanese equivalent of the Gap (sidenote: bought $10 jeans there on Saturday!) The name is written in カタカナ because it's a store, and store names are usually written in カタカナ. Additionally, カタカナworks well with ユニクロ's logo, because it is based off of squares, and the katakana in ユニクロ has a highly geometric effect. After interacting with a native Japanese speaker, I now realize that ユニクロ is an abbreviated way of saying "unique clothing", which is a foreign phrase. I should have recognized this to begin with, but it's good that I've learned this now.
A confusing katakana word that I found is ゴミ, or the Japanese word for "trash". It's not a foreign word, so it's peculiar that it's pretty much always written in カタカナ. My best guess is that it is since ゴ ミ falls under the category of words that are used for emphasis. I ended up doing research online, and there was an explanation that referred to a sense of Japanese pride--essentially, Japanese people would prefer to act as though trash is not of Japanese origin, and therefore they use カタカナ to separate it from their native culture. Additionally, if you search online there is a scathing てがみ that someone wrote to a にほんごのしんぶん that stated that "gomi" is supposed to be written in hiragana, and newspapers only write it in katakana for the shock value. It's certainly an interesting insight into Japanese culture!
When textbooks explain katakana, the ones that I've read-- most notably Kana de Manga, a relic of my middle school days-- they explain it as the alphabet that is used for generally foreign words. This neglects to mention the many, many exceptions there appear to be to this general rule. For instance, according to more thorough sources, certain non-foreign medical terms are written in katakana. Additionally, some non-foreign company names are written in katakana.
