I posted this in response to a similar query last year.
It's got to be one of five broad categories.
(1) They earn vastly more than you think (dishonest about cash-in-hand work, or simply a bit cagey about actual, genuine salary?). Probably unusual, and surely difficult to hide, and may not last.
(2) They are extremely frugal. Very difficult - and anyway lots of people are, and are still skint. There's a limit to how far you can "cut back" and yet still have expensive cars, clothes and holidays. Savings on value tins of tomatoes aren't going to pay for all that.
(3) They have a tidy amount put away in savings over the years. Possible, but again won't last for ever.
(4) They buy far more on credit than you are aware of. Quite likely, but will eventually come round to bite them.
(5) They have some hitherto undisclosed source of income - bank of Mum & Dad (especially for holidays), inheritance from grandparents or great-aunt, secret lottery win. Likely, and again not a bottomless pit.
My feeling is that it's almost always far more likely to be (4) and/or (5) than any of the others. They're the options which people are least likely to own up about, and indeed they may wish to give the false impression - either by implying it or saying so directly - that it's a bit of (1) and a bit of (2), because that feels more "earned".
Ask yourself - if it was you with option (5), would you keep it quiet? I know I would.
I agree that it is extraordinarily frustrating when you don't know, because then you start to suspect nefariousness. But you can understand why they would not want to let on about (5) especially, because they might imagine it could create bad feeling or awkwardness if known about.
Or (6), she sells her body, and/or her pants on the Internet.