You asked a slightly different version of this question on meta. For the sake of completeness, I'm going to largely answer this question by cutting and pasting material from that one. This is an interesting question because it allows for exploration of a larger topic: how do "the authorities" know the transaction had happened? Would the IRS, if they had any stake in this at all, actually know about the money? Questions like this often come up as a result of IRS Form 8300, which is the vehicle by which FinCEN is alerted of possible money laundering transactions. Filings of this form are sometimes called CTRs (currency transaction reports). People ask questions related to this process often, out of interest in knowing how or why transactions are reported to the IRS.
However, the thing people often forget is that only cash transactions are required to be reported via form 8300 - and only if they are over $10,000, or multiple transactions are made that total over $10,000. The goal of this regulation is to provide a method to capture transactions which involve moving funds in to or out of the (electronic) financial system. Changing funds back and forth between electronic and physical currency is often an important step in money laundering operations (because - while electronic funds leave a trail, paper money can be hard to track). Hence, the government needs a reporting mechanism in order to allow tracking of these transfers.
Your scenario of an electronic transfer would not, by default, strictly require reporting on a CTR. So, in the strictest sense, there's not strict regulatory reason why the IRS would know about this transaction.
That said, financial institutions are also required to report to FinCEN (and by extension the IRS) anything they consider suspicious. This type of reporting is often referred to as a SAR (suspicious activity report). This is somewhat open ended and there are not perfectly strict rules around what counts as suspicious, but generally most institutions look at patterns of behavior to determine what to report. Some even have automated reporting tools that pull potential transactions. A married couple with typical middle-class-American transaction volumes on a checking account (say, paychecks deposited every two weeks and several thousand dollars of debit card swipes, bill pays, etc to spend it back down) who received an 8-figure electronic transfer would almost certainly trigger any automated or human-decision flagging as suspicious activity.
This kind of "profiling" is related to a regulated practice called Know Your Customer (KYC). Know Your Customer provides a set of standards by which financial institutions are required to verify identities of anyone they do business with, along with a set of (somewhat loose) guidelines for staff to basically keep an eye out for unusual activity (such as a couple with a few thousand in monthly transactions suddenly receiving $33M). The identity verification component basically means that the bank would know the identities of the couple (seems obvious, but the standards are much more rigorous than what had existed in the past) and would almost certainly flag this transaction.
So, despite the lack of requirement for a CTR, the answer to your question about when the IRS would know is almost certainly "immediately."
Perhaps the more important question, though, is what would the IRS do once they had that information? Based on my exposure to SARs, there is visible follow up on only a very, very small fraction of them. There are legitimate reasons to receive a large transfer which might not be illegal or even result in any tax burden (taking out a loan, receiving a gift).
As a slightly related footnote, in addition to the above, banks in the US are required to keep specific types of records about some electronic transfers (which would easily include your example), but these records are only required to be retained by the institution, not automatically supplied to the IRS. So these requirements don't directly impact your specific question, but the bank would be required to keep information on hand about this transaction above and beyond the basic record keeping they keep on all transactions.
Since this version of your question includes additional details about exactly how the money came into this couple's possession, I think we can continue your fictional exercise and come up with a better answer than "probably nothing will happen." Since we know that the $33M is dirty, we can assume that - somewhere within the criminal enterprise that created pile of money - there are chinks in the armor. We don't know the nature of the fictional criminal activity, but suffice to say, any activity large or complicated enough to generate $33M would likely end up on someone's radar at some point. So, the ultimate way that the authorities may arrive at the doorstep of this couple would likely be because they were following the paper trail through normal criminal investigations about this criminal enterprise. If we are assuming that the WKP is a rogue vigilante actor (and not actually involved in official law enforcement activities), then there's probably a strong argument that real law enforcement may not considered him to have ever been the legitimate "owner" of the money, and by extension may consider the couple in question to have no legitimate stake to the money. in essence, although your question seems to have a tax-based slant, it's probably more likely that the couple in question would end up in legal trouble vs having any outstanding debt to the IRS.
If the money was traced to the couple by law enforcement, but they were able to somehow dodge any criminal implications (perhaps the WKP thought that through and coerced the now-deceased criminals into somehow legitimizing his claim on the money), there would still essentially be a tax burden for the WKP. The IRS imposes a lifetime limit of $11.4M, after which the giver needs to pay tax on additional gifts. So, even if we assume the WKP has never gifted anything in the past, there would still be taxes due on a large portion of the $33M gift.
Further to all of this, personally, I think the true question here is one of morals, not laws. From my perspective, the couple in question is accepting a gift of what we know to be illegally obtained property. Even if the WKP had some arrangement with the criminals to make their ownership legitimate, it sounds like the criminals obtained that money by illegitimate means - by ripping off other victims. Personally, I would not be able to sleep at night knowing that I was gifted $33M in stolen money, even if I had lost a little money myself - after all, even if I give the money to charity, that still means that there are other innocent victims out there in the world who, like me, were taken advantage of. Why should I, by extension, take advantage of me? Why not try to work with law enforcement to compensate other victims?
