These two verses tell us a little more of the situation. First, the priests appear not to have been previously aware of this ritual dancing place, as verse 4 indicates that they "discovered" the daughters of the Lamanites there. Once they were "discovered," the priests apparently watch though more than a single ritual occasion. They do not act immediately, but rather only when "there were but few of them gathered together to dance."
In order for the priests to "discover" the women, they had to be in the right area, which means they had already made the decision to raid the Lamanites for women. They were probably looking for a time when women might be washing in rivers or some other activity that isolated them. They happened upon the ritual spot, which fit perfectly into their plans. Nevertheless, with their fear of both the now-Limhites and the Lamanites, there would be no reason to be in the area of Shemlon unless they had previously decided upon this scheme to raid women.
Textual: It is not clear at this point how Mormon has this much information about the priests. It may be that he simply takes Gideon's supposition, and based on later information about these priests understands it to be correct, and therefore enters it in the record. Certainly there would be no contemporary record in Limhi's (or Alma's) possession that might be used as Mormon's source among Nephite records. The general facts of the abduction would have been available, however, from the conversation with the Lamanite king. Mormon appears to be using his position as historian to provide a more firm description of events than would have been available contemporaneously.
Social: While twenty-four women were taken, we cannot be precisely certain that this was a single woman for every man, though that is most likely given the hardship of the land. However, these priests had been used to polygamy if not concubinage, and might have preferred to have more women to do more of "women's work" which in agricultural societies was very important to survival. It is tempting to see twelve priests each taking two wives. This would suggest that they followed the Israelite penchant for honoring the twelve tribes, and had replaced Alma as a member of their group to return the number to twelve after his departure.