[Rusty]

The usages of 'se' are varied. Rather than discuss all of the different uses, which can be found here in the forums or elsewhere online, I'll simply identify the two usages employed in the clauses you cited.

The first clause is an example of the passive 'se' construction. The direct object in the active voice (las muellas) becomes the agent in the passive and the verb is always a third-person conjugation, preceded by 'se'. If the agent is plural, the verb is plural; if not, the verb is singular.
In the active voice, the model is 'tomarle algo a alguien' (take something from someone). They took fingerprints from her (they took her fingerprints).
In the pasiva refleja (passive 'se') construction, the English passive voice equivalent is 'Fingerprints were taken (from someone).' The 'le' in both the active and the passive is an indirect object pronoun that refers to the girl (referent appearing in a sentence previous to your example) from whom the fingerprints were taken.

The second clause is impersonal in nature; no particular person (or persons) is performing the action.
In an impersonal 'se' construction, an intransitive verb is used (ir) and it is always a third-person singular conjugation. The verb dejar, in this case, is an auxiliary to the verb ir. The meaning is 'allow/let'.
The model is 'dejarlo/a hacer algo' (allow him/her to do something). They allowed her to go home (they let her go home).

(The verb 'dejar' is versatile. It can be an auxiliary verb, whose meaning has already been explained, or it can be a transitive verb, taking both a direct object and an indirect object. In this sense, the verb takes on the meaning of 'bequeath.')