Thank you for your question.
To prevent money laundering, any transaction that adds up to $10,000
(be this one check or cash amount or several amounts that seem to the
bank employee to be done solely for the purpose to not be reported as
a $10,000 sum, such as two deposits of $5,000 or four deposits of
$2,500) is reported to a government agency, which can report it to the
Wikipedia entry-- Money Laundering
'In the United States, for example, cash transactions and deposits of
more than $10,000 are required to be reported as "significant cash
transactions" to the Financial Crimes Enforcement Network (FinCEN),
along with any other suspicious financial activity which is identified
as "suspicious activity reports".'
Bank employees are required to report this activity when they see it.
FINCEN can report the suspicious monetary activity to many different
government agencies, including the IRS.
Wikipedia entry-- FINCEN
"Their primary purpose is to gather information on the movement of
large or suspicious amounts of money, and to increase the
communication about that movement to various domestic and
international law enforcement agencies, including the Bureau of
Alcohol, Tobacco, Firearms, and Explosives, the Drug Enforcement
Administration, the Federal Bureau of Investigation, the Secret
Service, the Internal Revenue Service, the Customs Service, and the
U.S. Postal Inspection Service."
Whether the IRS will audit you or not is dependent on whether FINCEN
reports the suspicious activity to the IRS. If the IRS gets reports of
your bank account receiving large amounts of money that have not been
reported to them or from which taxes have not been paid, they would
certainly proceed to auditing. FINCEN or the FBI might conduct an
investigation if large amounts of money are going through your
account, on suspicion of money laundering.
money laundering wikipedia
If you need any additional clarification, let me know and I'll be glad
to assist you.