I'd say he was hiding money, but then my ex did this during our (very protracted) divorce.
Initially, he drew out £6k in cash from his savings and paid it into his sister's building society a/c. The thick twat left the counter receipt among papers on the desk we both used, and it clearly showed the first 5 letters of her unusual surname and her initial. But I was a thick twat too, and challenged him about it, instead of using it to discredit him later in court.
He always used to draw out £200 a fortnight in cash from his own money, for beer, cigs, lunches etc. That went up to £300 a fortnight, then every 10 days, until after a while he was taking out £300 a week. He only went to the pub at weekends, so he was definitely squirrelling a fair bit of it away somewhere for the next 3 years.
By the end, I couldn't really be arsed to make much of it, as I was just desperate for the divorce to be over.