I doubt there is someone stupid enough to keep cash in deposit boxes - this is kenya - the anti-money laundering simply don't exist - we don't only clean our money - we clean dirty money from Congo, South Sudan, Somalia, Nigeria and name it.
Any dirty money in kenya is cleaned easily. What people keep is transaction money - a few millions here and there - for bribing and the ones they receive as bribes - before it get cleaned.
I know you're interested in Ruto...if Ruto had so much money...he would do what Mois, Kenyattas, Mwaus, Biwotts do...just buy or form a small bank whose main business is to clean the money. Ruto obviously has interests in insurance...and several other business..like real estates...that easily clean up his loot.
The way forward is to demonize - we have Mobile Money (MPESA) acting as legal tender already - CBK should be working towards digital money ( I mean Zimbwabwe has done it) - and that would be more interesting fundamental change in our payment systesm and would spark more innovation, regulation and name it - than the usual Ruto bla bla de bla.
There must be quite some big thieves and druggies who will be smoked no doubt. The kind who stash in deposit boxes or private vaults. Cash is good for netting more taxes not graft -you're right tax evasion is a bigger problem than graft. Cashless + AI can nail lots of billions.