I think this statistic is related strictly to infants in the first year of life. Since 1983, there have been 278 infants abducted....and all but 12 have been recovered. Almost half of these 278 were taken from hospitals.
The figure for abduction of all children is much higher. In 1999, there were 50,000 non-family child abductions in the US....just to throw a figure out there.
You're right! 278 in 30 years is infants for stereotypical (stranger) abductions.
For all children under 18, it's 797,500 a year, of which only 115 are stereotypical (stranger) abductions.
Dept of Justice stats via NCMEC:
http://www.missingkids.com/en_US/documents/Statistics.pdf
Here are the infant stats, that show the 278:
http://www.missingkids.com/en_US/documents/InfantAbductionStats.pdf
Nonfamily Abducted Children: National Estimates and Characteristics
Defining Nonfamily Abduction and
Related Terms
• Nonfamily abduction: (1) An episode in which a
nonfamily perpetrator takes a child by the use of
physical force or threat of bodily harm or detains
the child for a substantial period of time (at least
1 hour) in an isolated place by the use of physical
force or threat of bodily harm without lawful authority or parental permission, or (2) an episode in
which a child younger than 15 or mentally incompetent, and without lawful authority or parental permission, is taken or detained or voluntarily accompanies a nonfamily perpetrator who conceals the
child’s whereabouts, demands ransom, or expresses
the intention to keep the child permanently.
• Stereotypical kidnapping: A nonfamily abduction
perpetrated by a slight acquaintance or stranger in
which a child is detained overnight, transported at
least 50 miles, held for ransom or abducted with
intent to keep the child permanently, or killed.
• Stranger: A perpetrator whom the child or family do
not know, or a perpetrator of unknown identity.
• Slight acquaintance: A nonfamily perpetrator whose
name is unknown to the child or family prior to the
abduction and whom the child or family did not know
well enough to speak to, or a recent acquaintance who
the child or family have known for less than 6 months,
or someone the family or child have known for longer
than 6 months but seen less than once a month
https://www.ncjrs.gov/pdffiles1/ojjdp/196467.pdf