On the question of what might have happened to Nembo . . . he might indeed have encountered a predator. It is not impossible for that predator to be a peregrine, although it is not common.
A few years ago at Rochester, there was a family of kestrels fairly close to Mariah and Kaver's nest. The kestrels used to buzz the falcons, who ignored them. However, one day the male kestrel was buzzing Kaver, and Kaver suddenly turned in mid-air and caught him.
Poor little kestrel. Mariah and the babies wanted Kaver to share his prey, but he would not.