The rules don’t define this. The absence of anything saying your soul goes anywhere might indicate that it stays where it is, though—in fact, the concept of selling souls isn’t explicitly mentioned in the warlock description at all. The only use of the word “soul” in the Classes section of the Player’s Handbook is for the monk’s Diamond Soul. Actually, selling one’s soul at all is only mentioned as a possible Bond for the sage background—and it’s not given any details there, either, it just says “I sold my soul for knowledge. I hope to do great deeds and win it back.”
Anyway, since one can sell one’s soul, and the warlock description emphasizes the search for secret knowledge and everything, it seems entirely plausible that one’s soul could be one’s side of the bargain in a pact—certainly tons and tons of narrative precedent there. So we’ll say that’s what happened—so what does that mean?
Again, we have no details—it’s up to you. Traditionally, though, sold souls are collected on death, not before. For that matter, soulless characters are often not depicted as independent, sapient beings—which would make them not really viable as player characters. But as DM, you can make the warlock’s soul absent. Just be aware that this will have ramifications beyond whether this seer can see the warlock. For a fairly-obvious example, magic jar would presumably have no effect on the warlock. There will likely be others.