The total amount of us currency in circulation divided by the us population comes out to about $3,500 per person. that is more than most of us carry. where is all the cash?

Some of the money is stored in banks - those are called reserves. Banks usually keep a portion of the money that people deposited in their reserves.

Additionally, some cash is in the cash register - every cash register has some money to be able to give people change - so this is money owed by companies, and not by private people