Well Estonia allows 2 passports (but NO dual citizenship) AND an ID card. 2 passports because every country outside EU requires us to obtain a visa. So I always travel with 2 forms of identification. However the passport that's missing has a few important Visas for me which makes it quite important for me to get it back. Apparently there is even a reference number somewhere. Don't know what to do with it. Wrote an email to San Francisco Airport lost and found as a starting point.