Microfilmed card indexes are a great finding aid for genealogists. It's worth remembering that when thousands of cards are sorted by humans, errors are bound to occur.
When using microfilmed card indexes, I always "browse" in the vicinity of the name for which I am looking. The reason is this set of cards from the Illinois, "Northern District Naturalization Index, 1926-1979" on FamilySearch.
The cards were filmed in the order in which they were microfilmed. The composite image below shows cards that were digitized as image 440, 441 and 442 respectively. I was looking for Peter Verikios. In browsing, I navigated my way to image 440-George Verikios. When image 441 was a different last name, I almost quit.
Good thing I didn't.
There after Frederick Harold Verinder's card was the card for Peter Verikios. Clearly Peter's card was out of order.
Always browse the region of indexes of this type. Often these sets of digital images are not high on the index priority as they are already indexed.
A little browsing may go along way to helping you find your missing person.