The database is "Registration Certificates - Widows, Divorced Women, & Monors, 1907-1914." The database is not really "hidden," of course. It is just one of so many that unless you know it's there, you will never think to look for it...and it's a relatively new one.
It's true that if you do a general person search on Ancestry, this database will be culled for your search criteria, but the results may be quite a few pages down the hit list. In general, if you think a particular database will be productive, it may be a good idea to search it specifically. To do that, click on the search tab, then select "catalog" and then enter words from the title or a key descriptive word.
OK, so after all that, what does "Registration Certificates" give you. Allow me to quote from Ancestry:
certificates in this database were generated by U.S. citizens living abroad who
were registering their intent to retain their U.S. citizenship. The registrants
were widows, divorced women, and minors who had been born abroad to parents who
were U.S. citizens. Registrants used several different forms, including
separate forms for women who had obtained U.S. citizenship via marriage and
women who were citizens before marriage. Information contained on the
certificate can include: name, birth date, birthplace, marriage date, spouse’s name, and spouse’s birthplace. Records
may also list death date (spouse), marital status, residence, date a person
left the U.S., dates of residence in foreign country, naturalization facts, and