to spy on someone, or to stalk someone (see Molina)
Alonso de Molina:
pipia. nite. (pret. onitepipix.) espiar, o asechar a otro. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 82r. col. 1. Thanks to Joe Campbell for providing the transcription.