I think you may be referring to Jehovah’s Witness on those two doctrines. But JW’s and SDA’s do both believe Jesus is the Archangel Michael. They also both taught that Jesus has already returned to earth (invisibly of course). They had to come up with some explanation for their false prophecies not coming true. I believe the SDA predicted Christ’s return in the 1800’s and the JW’s in 1914.
Adventists have never taught that Christ has come back in secret. They teach that the second coming is going to be massively unsecretive. It will seen by the whole world.
*** I think you may be referring to Jehovah’s Witnesses on those two doctrines. ***
No, that is why I linked to two Adventist Websites. Both JW’s and SDA’s have their roots in the Adventist Movement of, IIRC, the early 1800’s. So they do have some similar beliefs.