It is actually there. Even though grave sins forgotten and thus omitted at the time of
confession are* indirectly* remitted, the canon indicates that these too must be
confessed
directly. CIC Canon 988 - 1. A member of the Christian faithful is obliged to confess in kind and in number all serious sins committed after baptism and not yet
directly remitted through the keys of the Church now acknowledged in individual confession, of which one is conscious after diligent examination of conscience.
Also CCC give these sources for footnote 54: Council of Trent (1551):
Denzinger-Schonmetzer 1680 (ND 1626);
cf.
Ex 20:17;
Mt 5:28.
We see from the Council of Trent Session XIV, Nov. 25, 1551 A.D.:
CANON VII. – If any one saith, that, in the sacrament of Penance, it is not necessary, of divine right, for the remission of sins, to confess all and singular the mortal sins which after due and diligent previous meditation are remembered, even those (mortal sins) which are secret, and those which are opposed to the two last commandments of the Decalogue, as also the circumstances which change the species of a sin; but (saith) that such confession is only useful to instruct and console the penitent, and that it was of old only observed in order to impose a canonical satisfaction; or saith that they, who strive to confess all their sins, wish to leave nothing to the divine mercy to pardon; or, finally, that it is not lawful to confess venial sins; let him be anathema.
history.hanover.edu/texts/trent/ct14.html
When the faithful do not come forth with the remembered mortal sin, already resolved, they do not receive a penance for it. Another canon states that we are to receive a penence:
CIC Canon 981The confessor is to impose salutary and appropriate penances, in proportion to the kind and number of sins confessed, taking into account, however, the condition of the penitent. The penitent is bound personally to fulfill these penances.
Also even when there is general absolution given, with approval of the bishop (which means generally in wartime or a period of one month without opportunity to confess personally), we still must have the intention to confess personally. In that way we receive our penance. See CCC 1483, CIC 961.2.
Can. 960 Individual and integral confession and absolution constitute the only ordinary means by which a member of the faithful conscious of grave sin is reconciled with God and the Church. Only physical or moral impossibility excuses from confession of this type; in such a case reconciliation can be obtained by other means.
Can. 961 §1. Absolution cannot be imparted in a general manner to many penitents at once without previous individual confession unless:
1/ danger of death is imminent and there is insufficient time for the priest or priests to hear the confessions of the individual penitents;
2/ there is grave necessity, that is, when in view of the number of penitents, there are not enough confessors available to hear the confessions of individuals properly within a suitable period of time in such a way that the penitents are forced to be deprived for a long while of sacramental grace or holy communion through no fault of their own. Sufficient necessity is not considered to exist when confessors cannot be present due only to the large number of penitents such as can occur on some great feast or pilgrimage.
§2. It belongs to the diocesan bishop to judge whether the conditions required according to the norm of §1, n. 2 are present. He can determine the cases of such necessity, attentive to the criteria agreed upon with the other members of the conference of bishops.
Can. 962 §1. For a member of the Christian faithful validly to receive sacramental absolution given to many at one time, it is required not only that the person is properly disposed but also at the same time intends to confess within a suitable period of time each grave sin which at the present time cannot be so confessed.
§2. Insofar as it can be done even on the occasion of the reception of general absolution, the Christian faithful are to be instructed about the requirements of the norm of §1. An exhortation that each person take care to make an act of contrition is to precede general absolution even in the case of danger of death, if there is time.
Can. 963 Without prejudice to the obligation mentioned in ⇒ can. 989, a person whose grave sins are remitted by general absolution is to approach individual confession as soon as possible, given the opportunity, before receiving another general absolution, unless a just cause intervenes.
There is a slight difference in the eastern canon law (CCEO).