"Se" is one of the trickiest words in Spanish, because it has more functions than just indirect object. Let me list you the most common: ⇒Indirect object 3º person singular when accompanied by a direct object pronoun. Ex: no le des el libro a la chica > no se lo des. ⇒Reflexive. This is similar to "oneself" in English, but there isn't a 100% correlation. Ex: Pedro estaba cocinando y se quemó (he burnt himself). ⇒Reciprocal. Similar to "each other". Ex: los dos novios se envían cartas de amor. ⇒Pronominal. In this case, the suffix -se is attached to the end of the verb in the infinitive form and must be used with the appropriate pronoun when conjugated. For example, "arrepentirse" (to regret) must always be used with the pronoun, so you'd say: me arrepiento, te arrepientes, SE arrepiente... ⇒Impersonal. In this case, the sentence becomes neutral and loses the subject. These types of constructions are often used when you say how something is done. For example: cuando un perro es muy pequeño, no SE sabe si es macho o hembra. Estás sujetando mal el bate de béisbol, SE coje así. ⇒Pasiva refleja. This similar to a passive voice in English. Example: en esa tienda se venden gorros (hats are sold). In the examples you provided, we could say that "cómo se dice" uses an impersonal "se" and "se busca" is a pasiva refleja (someone is wanted). Please, ask if you have questions!

Because it is an impersonal sentence. It doesn't matter who says it. Most of these sentences are are translated into English in a passive. Se dice que la insegurida ha aumentado ==== It is said that insecuity has increased. In the case of "se busca" is very similar.

We do a similar thing in English. For example, we don't say "how say X?", instead we say "how do you say X?" or "how does one say X?" or "how is X said?" etc. The 'se' in '¿cómo se dice?' functions in a similar way: How do you say?, How is it said? How does one say? etc.