So what? If you don’t notice it, you don’t notice it. If you make attempts to contact the people who made a mistake, you’re making a reasonable effort. That doesn’t make purposely keeping it and hoping no one notices all okey dokey.
On an unrelated issue, I was just looking up conversion last month, and couldn’t find anything in my state criminal law. I asked my lawyer, who practiced in my state, and has since moved to another state (prosecutor), and she’d never heard of it. Evidently conversion is not a crime in vic.aus or nsw.aus. I’d only come across the description in old general accounting texts, not local criminal law.
Not making a point, just a curious observation.
Electricity companies in vic.aus. You are protected by a law that says they can’t come after you more than two years later, for mistakes they made in billing. As it turns out, if they overcharge you, and argue about it for two years, and then admit their mistake… it’s too late. They aren’t going to pay you back, because it’s more than two years.
Generally, in common law jurisdictions, conversion is a civil tort and not a criminal offense. However the common law world is rather large. I wouldn’t be surprised if some jurisdiction somewhere had a criminal offense called conversion.
In my jurisdiction, it may only be codified in connection with specific property. Unlawful use of a motor vehicle, for example, is not theft because it doesn’t require proving an intent to permanently deprive the owner of the vehicle. Some of the identity theft statutes are also a species of criminal conversion. But I don’t think we have a general criminal conversion statute.
Remember though, that the bank is not the owner of the money. So leaving money in your account and saying nothing to anyone does not leave it in a place where the owner has free access to it.
As far as other kinds of property, what is reasonable would depend on the circumstances. If you had friends over, and you later notice a 1 carat diamond ring laying on your porch, partly under a large planter, doing nothing to notify the owner of the Lost property and just waiting for someone to come and get it seems pretty unreasonable to me. But your car example seems fairly reasonable as far as it goes.
I should think that the extra Whopper is punishment enough. 
-MMM-
Something like this happened to me years ago. This was in the days when I did my bill payments at an ATM instead of online.
Payday at my old job rolled around. My pay was deposited by direct deposit. After work I went to the ATM on the corner and paid my bills, then looked at the account balance. Instead of the couple of hundred dollars I was expecting, there were over 1400 dollars in the account!
The next day I went to HR and asked what was going on. Turns out they had double-paid a whole bunch of people. They decided that the easiest thing to do was just leave the extra payments there, and announce that the affected people had gotten their next pay two weeks early. 
I guess that counts as ‘reasonable effort’ to address the situation…
This happened to a friend of mine in the late 90s. One day he noticed his brokerage account had an extra $300k in it. He knew enough not to touch the money, but decided it was too much trouble to try and contact the brokerage. This was in the early days of discount internet brokerage firms, and customer service was not one of their strengths. We had fun checking every few days to see if the money was still there, and after a few weeks it vanished. I think he got a letter from them at some point, or perhaps it was just a statement, marking the deposit as erroneous, and the withdrawal as a correction. There were never any repercussions.
My sister worked for an administratively challenged private school in the mid 1990s. After she quit, she still continued to get direct deposits every week. They said they couldn’t reverse the transactions and wanted her to write them a check for the GROSS amount, saying that they had paid the taxes and insurance deductions already and couldn’t recover them.
She told them to FOAD and wrote them checks for the amounts actually deposited. They ended up actually suing her for the difference (~$1000). She ended up paying a lawyer an couple hundred bucks who convinced them that to drop it.
Then they issued her W-2s showing the returned payments as income. She filed with the correct amounts but had to deal with the IRS, employing a CPA/EA which cost another couple hundred.
As for “reasonable measures” I don’t think it is up to me to fix someone else’s screwup especially if I leave it in my account to let them deal with it.