From R. Daniel Z. Feldman, The Right and the Good: Halakhah and Human Relations, pp. 145-146 (buy the book):
Were a waiver of claims the only goal of the [forgiveness] process, it would follow that if the victim would forgive of his own initiative, without waiting for his oppressor to seek his pardon, the latter gesture would become redundant. Nonetheless, many authorities who concern themselves with this issue indicate that a request for forgiveness is necessary even if the other party has already excused the offense. R. Binyamin Yehoshua Zilber, among others, maintains that the obligation to seek mechilah is operative regardless. However, R. Yehoshua Ehrenberg is inclined to believe that unrequested forgiveness is enough.
A story related by the Talmud is cited by those who agree with R. Zilber as support for their position. Rav had been offended by a certain butcher, and, following the passage of some time, they had still not reconciled. As Yom Kippur was approaching, Rav took pains to make himself available to the butcher so that the latter may apologize. R. Yitzchak Blazer observes that in doing so, Rav was engaging in a form of imitatio Dei, as God also brings Himself closer to facilitate repentance during the Ten Days of Penitence between Rosh Hashanah and Yom Kippur. That aside, the very necessity of accessibility on the part of Rav is troubling; as he is clearly prepared to forgive and forget, there should be no need for the butcher to ask. It seems, then, that the act of apologizing is integral to the forgiveness granted on Yom Kippur. Similarly, R. Eliezer Ginsberg writes that the mechilah would be ineffectual, lacking genuine penitence on the part of the sinner.
This element is relevant to another issue of concern among authorities. Yom Kippur, mentioned as a motivation to seek mechilah, is seemingly superfluous; if an offense has been committed, forgiveness must be sought irrespective of the time of year. R. Ephraim Zalman Margoliyos, in his classic collection of the laws relevant to the High Holy Day period, Matteh Ephraim, writes that this is, of course, the case; however, Yom Kippur is noted as the final deadline for this obligation. R. Pinchas A. Z. Goldenberger suggests an approach in line with this. If an interpersonal violation is committed, pardon must be sought immediately; nonetheless, if the victim bears no grudge, then this action is of less necessity. However, the impending arrival of Yom Kippur imposes an additional requirement of obtaining mechilah that is not suspended in the event of unsolicited forgiveness.
 Responsa Az Nidbaru 2:65.
 Responsa D’var Yehoshua 5:20.
 Yoma 87a.
 Kokhvei Ohr 5.
 Isaiah, ch. 58, as per Yevamot 49a.
 See also R. Shlomo Zalman of Volozhin’s Toldot Adam.
 V’Atah B’Rachamekha HaRabim, Hilkhot Teshuvah 2:9.
 Matteh Ephraim, 606.
 Responsa Minchat Asher 3:32.
 See the similar interpretation in R. Moshe Shternbuch, Responsa Teshuvot V’Hanhagot 2:285.