You just need to look at the language being used in these late stages - headlines like "save the union" have been common and will only intensify. If they have to work so hard to save it, do they genuinely believe a YES result being likely.
Also - people, perhaps subconsciously, talk about Scotland "winning" independence. No one ever talks about "winning" the Union.
And not being too paranoid, but I hope the YES vote is decisive so counter any potential sneaky tricks that may occur.