Obsessing isn't worth the energy, but protecting ourselves is.
My xH stole money from his family (us, his wife and children) to fund his habits and try to build a private nest egg, and I believe it's fairly common for addicts and abusers, etc., to steal from their family regardless of their income level.
Prepaid cards can anonymously hide or launder all kinds of money and activities. Many people are using them to funnel money from joint partnerships (including marriage) in preparation of a foreseeable dissolution of that partnership. Trustworthy partnerships are transparent in all matters, especially money. A partner hiding things is good reason not to trust them, since by that act they have proven themselves untrustworthy.
We could drive ourselves crazy trying to control any or all of the possible actions of others or how someone is using hidden money, or, we can accept it for what it is and let them go about their business as usual, while we keep our bearings and consult with a very reputable financial adviser on how best to protect ourselves financially.
Letting go for me was a process and it took a number of undeniable things piling up to snap me out of the fog. Taking steps to protect myself was very empowering, and with each step, what "they" did mattered less and less.
...and how very true, more is always revealed...