Before I answer this, I need to tell a story.
In 1978 or 1979, when I was 16 or 17 I found a large sum of money and turned it in. It was the last shopping day before Christmas and I had gotten on a bus about an hour before stores closed. There was only a couple of people at the front of the bus and being a typical teen, I went towards the back of the bus to sit down. After sitting I looked across the aisle and saw a large paper shopping bag like the kind you used to get in department stores. It looked like it had something inside it so I got up and went and looked. Inside was a seashell and straw purse: typical granny style.
The purse was full and included a wallet with identification, credit cards and over $700.00 in cash: Likely the amount of the ladies monthly pension cheque. To give you an idea of how much money that was at that time, minimum wage was $1.85 per hour.
I took the bag and purse to the bus driver and reported that I had found it. The supervisor was stationed 2 stops ahead, so he in turn reported it to the supervisor. All 3 of us took inventory of the purse and wallet before the supervisor took the purse for the lost and found. They took my name, address and phone number to give to the owner of the purse when she picked it up.
My brother who is a bus driver checked on the status of the purse a couple of days later. The woman had picked up the purse and was grateful that it was returned completely intact. She was given my name, address and phone number.
I wasn't expecting a reward! However, a telephone call to thank me for returning her purse with all of the money inside would have been very welcome. I'm 49 years old, and to this day the fact that she didn't pick up the telephone and thank a 16 year old teenager who was honest enough to turn in not only her purse, but all of her money, still burns me.
So if I found $100,000 in boxes sitting inside a tire by a dumpster, would I return it? Not on your life!
However, as Sarah pointed out, if you started spending large sums of money or sticking huge windfalls in the bank suddenly, people would become suspicious. So what I would do is basically cook the books so to speak.
I would again start up my home based website building. Make up invoices for phoney clients and take payment from my stash of cash. In time the money would find its way into my bank account. Now granted because I was claiming I was earning that income in a home business, I would have to claim the income on my income tax. However, I would be able to offset some of it with deductions and even more by putting it into tax free RRSP savings for my retirement.
In the end I would still be able to buy that car, take that trip etc.