Can it be sensed by beings other than humans with which humans might communicate? (Or does that already count as a method, however mediated by these alien beings, of sensing by humans?)

If somebody made this assertion, then I would, as MugaSofer, probably follow up with how this person came to that belief. Somewhere along the way there is likely to be some sensation by that human, whether a past sighting that can't be repeated, communication with (what the believer takes to be) alien beings (space aliens, fairies, angels, etc), or even just a vague feeling that a dragon is there. We might get meaning out of it (and then it is very likely, unless interpreted in a figurative sense, to be very likely false).