Learn Swedish: Någon hade låst dörren utifrån. - Someone had locked the door from the outside.

It is built by combining ut (out) and the suffix -ifrån (from). In Swedish, adding -ifrån to spatial adverbs like in, ut, upp, or ner creates a single new word describing where an action originates. So utifrån literally means "from the outside".

Could I say från utsidan instead of utifrån?

Yes, från utsidan (from the outside surface/area) is grammatically correct and means basically the same thing. However, the compound adverb utifrån is much more concise and natural in everyday Swedish. Just remember that you cannot say från ute, as Swedish doesn't combine the preposition från directly with the location adverb ute.

Why does this sentence use låst instead of låste?

Because it uses the past perfect tense ("had locked"). When you use the helper verb hade (had) or har (have), the main verb must be in its supine form, which is låst here. Låste is the simple past tense ("locked"), which you would use if there were no helper verb, as in: Någon låste dörren (Someone locked the door).

I've seen någon translated as "anyone" before. Why does it mean "someone" here?