Short answer. An illegal detention is 'serious' under Article 267 of the Revised Penal Code when any of four aggravating circumstances is present: the detention lasts more than three days; it is committed simulating public authority; serious physical injuries are inflicted or threats to kill are made; or the victim is a minor, a female, or a public officer.
What the law says
Any private individual who shall kidnap or detain another, or in any other manner deprive him of his liberty
Revised Penal Code, Article 267 — Kidnapping And Serious Illegal Detention. Read the full provision →
Serious versus slight
Illegal detention is the crime of a private individual depriving another of liberty. The law splits it into two grades. It becomes serious illegal detention, punished under Article 267 of the Revised Penal Code, when the deprivation of liberty is attended by any of the aggravating circumstances the article lists. Without any of those circumstances, an unlawful detention is treated as the lighter offense of slight illegal detention. So the presence or absence of the listed factors is exactly what moves a case from the lighter grade to the grave one, and it is why the same base act can carry very different penalties.
The circumstances that make it serious
Article 267 punishes Any private individual who shall kidnap or detain another, or in any other manner deprive him of his liberty when the detention involves any of four circumstances: 1. If the kidnapping or detention shall have lasted more than three days. 2. If it shall have been committed simulating public authority. 3. If any serious physical injuries shall have been inflicted upon the person kidnapped or detained; or if threats to kill him shall have been made. 4. If the person kidnapped or detained shall be a minor, except when the accused is any of the parents, female or a public officer. Any one of these is enough; they need not combine.
Ransom and the gravest outcomes
Two further situations push the crime to its most severe level. First, the article says The penalty shall be death penalty where the kidnapping or detention was committed for the purpose of extorting ransom, and this applies even if none of the four circumstances above is present. Kidnapping for ransom is thus serious in itself. Second, When the victim is killed or dies as a consequence of the detention or is raped, or is subjected to torture or dehumanizing acts, the maximum penalty shall be imposed. One caution on penalties: although the article still names death, Republic Act 9346 (2006) prohibits the death penalty, so in its place the courts impose reclusion perpetua.
Who is bound and what it does not cover
This crime is committed by a private individual, not a public officer making an official arrest; abuses by officers fall under separate provisions on arbitrary detention. A parent taking custody of his own minor child is expressly outside the minor circumstance. It also does not cover brief, momentary restraints that are really part of another crime, such as a robbery. The heart of the offense is a genuine deprivation of a person's freedom of movement, and it is the listed circumstances, length, deception, injury, threats, or the victim's status, that decide how heavily the law responds.