I entered the last four digits of my Social Security Number and provided basic date of birth and full name. Then it asked me three security questions that only I would know the answers to. For example, it asked where I was in 1989-1990 and it asked where something was.
Derived from information in public data records, ExpectID IQ serves up non-intrusive, intelligent questions relating to that person’s history such as something involving a previous address or an associated person.
This data usually comes from an ID verification process offered by credit bureaus in various countries. For instance, Precise ID from Experian or ID Manager from Transunion. Other companies like Trulioo may provide verification services globally. The credit bureaus themselves use them before you can access your own credit report.
The way these services work is that you supply some information to identify yourself and the service extracts personal information from your credit report (Airbnb does not receive your credit report, as this is not a credit check). They may use other public records databases as additional sources of information. They then ask you questions about this information: what bank do you hold a mortgage with, what address did you use in the 90s, etc... These are typically multiple choice questions with several fake answers intended to look plausible mixed in with the real one. I have seen some spectacularly bad fake answers though.
In some cases, this method is not able to provide adequate verification, such as if you have a sparse credit report with little history to ask about. In this case, they will fall back to requiring another form of ID verification. In some countries, Airbnb uses Jumio Netverify to facilitate scanning and verifying photo IDs for this purpose.
As a travel-related sidenote, the TSA in the United States can use a similar process for those traveling domestically without ID to prove their identity, though some airlines may still insist on ID, especially to check bags.