To make a reflex passive sentence you have to take the direct object, turn it into the subject, add a "se" and conjugate the verb in an active voice so it agrees with the subject. In the following sentence:
La gente usa verbos en varias circunstancias.
...the direct object is "verbos", so that will be your subject. Since "verbos" is plural, "usar" must be plural too. The sentence you get is:
Los verbos se usan en varias curcunstancias.
To make a question, the steps are the same. Therefore, ¿En qué circunstancias se usan? is a reflex passive voice.
The reason why passive voice exists is to shift the focus from the agent (the one doing the action) to the patient (the one receiving it). But, what if you don't have a direct object? Does that mean you have to specify the subject? Not necessarily. If there's no DO, you still can omit the subject, except that isn't called a passive sentence anymore but an impersonal sentence. Impersonal sentences are even easier to make: you start with "se", conjugate the verb always in singular, and you got it. Impersonal sentences contain intransitive verbs or transitive verbs with no direct object, and they have no subject. Passive sentences can be reworded as a standard passive sentence; impersonal sentences can be reworded by substituting "se" with "uno".
So far so good, but native speakers who use the language intuitively aren't generally aware of such technicalities. They may not even realise these constructions are different things. They see a "se" in both kind of sentences, think they are the same thing and use them indistinguishably, which leads to some bizarre constructions. For example, a speaker may take a transitive verb, use it in an impersonal construction and just throw a DO in there. The sentence ¿En qué circunstancias se los usa? is an example of such construction. That "se" is an impersonal sentence marker, but later on you stumble upon a DO pronoun ("los") which makes the verb transitive. Since impersonal sentences should only be used with intransitive verbs, the sentence ¿En qué circunstancias se los usa? sounds stilted and is best avoided. The sentence ¿En qué circunstancias se los usan? just doesn't make any sense.
As you can see, the pronoun "se" is the most versatile pronoun in Spanish. It has up to six different functions, and sometimes which one is being used isn't that clear, making it possible to construe a sentence in more than one way. The sentence No sé dónde se encontraron los libros can be understood in two different ways: you can consider that "se" a reflex passive marker or an ethic dative. In the first case, it means "I don't know where the books were found". Let's break it down:
La gente encontró los libros (active sentence: people found the books) => Los libros se encontraron (passive sentence: the books were found)
"Los libros se encontraron" can be reworded as "se encontraron los libros". "No sé dónde se encontraron los libros" means "I don't know where the books were found".
In the second case, it means "I don't know where they found the books". "Encontrarse" means the same thing as "encontrar", where "se" is an ethic dative. No sé dónde [ellos] se encontraron los libros means the same thing as No sé dónde [ellos] encontraron los libros.
The second interpretation is probably what most speakers would understand.
Finally, the presposition "en" in "en qué circunstancias" is idiomatic. Another idiomatic expression is "bajo qué circunstancias". "A qué circunstancias" isn't right in this case, but it may be the right expression in other cases such as "¿A qué circunstancias te refieres?". In that case, the verb "referirse" takes the preposition "a", so "a qué circunstancias" is correct. When "en qué circunstancias" is used as "under what circumstances", "en" is the right preposition.
