Many clustering models imply two kinds of earthquakes: spontaneous ones and those triggered by previous earthquakes. The pair-wise links from earlier to later earthquakes control the estimates of the clustering parameters. However, earthquake catalogs are limited in time, space, and magnitude, so that triggers of some cataloged earthquakes may be unknown. Thus some links are unrecognized and some triggered events appear spontaneous. Here we present a method for identifying such earthquakes and reducing the bias from missing links. We treat earthquakes probably affected by missing links as potential triggers, but we exclude them in evaluating modeled effects. We use an Epidemic-type Aftershock Sequence (ETAS) model to examine specifics. The most affected parameter is the proportion of spontaneous earthquakes. The most important missing links apparently follow earthquakes below the magnitude threshold, before the start, and outside the spatial boundaries of the catalog, in that order.