Star Trek: 10 MORE Alien Encounters Before First Contact
9. El Aurian
While it seems that the earliest official meeting between the Federation and the El Aurians took place during the Enterprise-B's maiden voyage, there are several holes in that theory. First, the two transport ships that they are on are Federation in origin, so this suggests an earlier form of first contact.
Second is in the form of Guinan who, though present on the S.S. Lakul, had also been present on Earth far earlier than this.
When thrown into the 19th Century, Lt. Commander Data encountered his shipmate, posing as a member of high society, fully aware of her El Aurian status. This is to say - the Guinan who appeared in San Francisco as a contemporary of Samuel Clemens knew who she was, what she was, and was in no rush to leave.
As there have been (to the best of this writer's knowledge) no historical headlines screaming 'EL AURIANS ARE INVADING!' - this suggests that they had, at one point, the technology to evade human detection. Guinan may have continued to utilise this technology when she moved as far as Los Angeles, establishing her bar Ten Forward, wherein she encountered Jean-Luc Picard (again) over one hundred years later.