From an informal session on September 3, 1965, summarized by Robert Butts (Book 4 of "The Early Sessions"):
(Seth had some very interesting comments to make when the conversation turned to flying saucers, and the recent rash of reports on such craft in the newspapers. The craft do exist, Seth said, and the action of governments in relation to them leaves much to be desired. I believe he used the word "pitiable" in this connection. Also, government people cannot bear to admit something they cannot explain. Seth said that nevertheless the government had a point when they tried to prevent public panic by denying the existence of such craft. A great danger at this particular time would be that an admission of the saucer’s existence would, strangely enough, serve to unite the far- left and far- right extremist groups in this country. These groups would unite in declaring the craft to be Russian, that they were much advanced over anything we have; this whole furor would create panic, Seth said. In six months such reasoning would not apply, because of impending developments in this country. This would presumably be because of the impending elections, though Seth didn’t say this in so many words. In answer to my question he said Barry Goldwater would not be one of the far- rightists to cause trouble. Nothing was said about the source of the saucers, their inhabitants, etc.)
Seth made several side references to UFOs in the various sessions, but never focussed on the subject in detail... I don't think that he saw the subject as vital. Overall, Seth stated that UFOs originate from different planes--some physical, some "mental." There was no one specific locus of origin for the craft. Those familiar with Seth's discussions of "planes" know that a plane is not geographically related to any place in our physical universe; hence, there is no specific "where" that these craft might come from, at least in physical terms that we understand.
I can't find anything in the Seth writings to validate the more conspiratorial ideas of mass abductions or sinister hybridization projects... No warnings of impending alien takeovers--or rescues.
So, why did Seth not delve into the subject in detail? We can only guess. But I suspect that any explanation that Seth might have given would have been lost on his contemporary audience, which saw the phenomenon in physical terms. They lacked the necessary mental vocabulary and reference points to properly frame the subject--and probably still do. Or, as Betty Hill's alien abductors reportedly her when she asked where, on their three-dimensional star map, their craft originated, "If you can't tell us where *you* are on the map, what good would it do to say where *we* come from?"