If you found a safe stashed with cash in the wall of your newly bought apartment, what would you do?

I’ll suggest you leave the money there for at least six months. If the house sellers come back to claim it, give it to them. If some thug comes looking for it, give it to him. Try to learn about the history of the house to see whether anyone used to live there is currently in jail. If so, that’s trouble. Don’t touch the money. If whoever hid the money might had died unexpectedly without informing any family members. In this case, you’re safe. After six months, get another safe and find another place to keep the money, and dispose the old safe altogether. If someone comes back asking for it, say you know nothing about it. Maybe the sellers took it or someone else could have gotten to it before you, a contractor, the home inspector, etc. So many people have come through the house, you don’t know who. Then go and buy reloadable American Express or Visa gift cards where you can refill with cash at a local store, like CVS – not the same one every time. Don’t load too much into the cards, maybe $1,000 at a time. Basically all your expenses from this point are paid with gift cards. Don’t carry too much cash with you. It’s dangerous. Use gift cards instead.
That being said, don’t run back to the safe too often. Take out $10k at a time, divide it up among your family members and each should have a safe place to keep the money.
For your job, don’t do direct deposit into your bank, get a paycheck for every pay period. When you deposit that check, attach a random amount of cash, less than your paycheck or a little higher but not much, so it wouldn’t seem weird.
Then every 4-7 months, deposit a random amount between $7k-$10k into your bank, never the exact amount like $7k or $10k, and never whole dollars. So it should be like few random figures above it. If you’re married and each of you have two bank accounts, that’s an addition $60k or so a year to pay toward your mortgage.
If you have a side business where you get paid with checks and cash, you can attach some more cash whenever you deposit.
Once you have a decent amount of money in the bank, either buy a couple rental properties or pay off your mortgage. Please consider it a gift and don’t squander it.