A lot of speakers can disappear on some content. Maybe there are some that can disappear on all content, if put in the right situation. I imagine a single dipole positioned correctly could project a perceived image far behind the speaker. I think I’ve experienced that on some occasions even with a single cheap bookshelf. The sound seemed to come from the speaker’s direction, but it definitely sounded like it was well behind the speaker. What’s really rare is when it really seems like sound is outside, way behind the speakers, and somehow clearly coming through your wall, as if the wall was just a thin cloth fake.
I had a pair of Klipsch RF-7s doing that at one time (they didn’t do it individually in mono). Overall I was disappointed with those, but at one point I would listen and look out the closed windows with amazement because it seemed like a concert was going on out there.
Also, for me the ability of the speaker to disappear seems reliant on my state. Sometimes my brain falls heavily for the illusion. Other times it’s not so easily deceived. This morning I came to work and listened to a setup that I had configured last night that was really wowing me. When I came in this morning I had a quick listen and the disappearing effect was underwhelming. I didn't have the luxury to listen for long, but after a few minutes it was starting to work better. For me, the effect grows as my ears and mind relax into it, especially if they're not disturbed too much by other sounds.