I have seen two queens in one hive more often than i ever expected to.. So when you look for cells again look for eggs and larvae..
If you don't see any, take that frame your about to add, find the youngest larvae, the ones closest to the eggs, that are actually smaller than the egg in the next cell, Notch that cell.. IE OTS queen rearing, and it will almost guarantee they will turn it into a queen cell.
Mel "notches" several cells with the flat of his hive tool, then crunches the larvae he doesnt want used. I use the corner of the hive tool, which will pull down about three cells. Squish the egg/larvae in the adjacent cells so you only have one viable cell notched. You can do that in three or four places if you like, or want more than a couple cells for other hives/nuc's.