Organized crime groups often use front companies to disguise the origin and destination of their illicit funds, and to integrate them into the legitimate economy. Front companies are businesses that appear to be engaged in lawful activities, but are actually controlled by criminals for money laundering purposes.
One reason why organized crime groups use front companies is to gain control over an entire sector of the economy, such as construction, transportation, or gambling. By using multiple front companies, they can create a network of interrelated businesses that can manipulate prices, evade taxes, and influence public contracts. This can give them a competitive advantage over legitimate businesses, and allow them to launder large amounts of money through seemingly legitimate transactions.
Another reason why organized crime groups use front companies is to comingle their illicit funds with those of legal businesses. By using the bank accounts of front companies, they can deposit cash from criminal activities along with the revenues from legitimate operations, making it harder to trace the source of the funds. They can also transfer funds between different front companies, or use them to pay for goods and services, creating layers of transactions that obscure the money trail.
References:
* CAMS Study Guide - 6th Edition, Chapter 2, Section 2.2, page 43
* A Guide to the 6th Anti-Money Laundering Directive, Part 2, page 9
* Money Laundering and Financial Crimes, Chapter 2, page 29

The customer's business activity as a domestic carpet cleaning company is not inherently suspicious, as it aligns with their registered business. However, the significant weekly transactions from an international reputable antique auction house raise red flags.
The sudden increase in the customer's initial international receipt from $15,000 USD to an average of $30,000 USD per transaction per week is also noteworthy. Such a substantial change warrants further scrutiny.
The fact that 90% of the incoming deposits are matched by cash outgoings to non-related individuals could indicate layering or structuring activities associated with money laundering.
Overall, the combination of large international transactions, increased receipts, and cash outflows to unrelated parties from an antique auction house is the most concerning aspect.
References:
Anti-Money Laundering Specialist (the 6th edition) resources.
ACAMS AML Glossary (definitions of terms related to AML).1
ACAMS AML Foundations Certificate (overview of AML concepts and responsibilities).2 ComplyAdvantage: 6th Money Laundering Directive (6AMLD) (details on AML regulations and penalties).3

According to the ACAMS CAMS Certification Study Guide (6th edition), one of the red flags for money laundering is the structuring of cash transactions to avoid reporting thresholds. Structuring is the practice of breaking down large amounts of cash into smaller deposits or withdrawals that are below the reporting threshold of $10,000 in the United States or equivalent amounts in other jurisdictions. Structuring is done to evade the detection and reporting of cash transactions by financial institutions to the authorities. Therefore, when an accountable institution identifies that a customer is engaging in structuring or other forms of cash transaction manipulation, it should file a SAR/STR to report the suspicious activity1 References: 1: ACAMS CAMS Certification Study Guide (6th edition), page 64.
Reference:
https://www.fdic.gov/regulations/examinations/supervisory/insights/siwin07/article03_connecting.html

According to the ACAMS CAMS Certification Study Guide, 6th Edition, one of the four pillars of an effective anti-money laundering compliance program is the training of appropriate employees on their responsibilities and the institution's policies and procedures. Training is essential to ensure that employees are aware of the risks of money laundering and terrorist financing, the applicable laws and regulations, the red flags and indicators of suspicious activity, and the reporting and record-keeping requirements. Without proper training, employees may not be able to perform their duties effectively and efficiently, and may expose the institution to legal, regulatory, reputational, and operational risks.
References:
* ACAMS CAMS Certification Study Guide, 6th Edition, Chapter 2, Section 2.1.4, page 39.
* ACAMS CAMS Certification Video Training Course, Module 2, Lesson 2.1.4.

This is a red flag of money laundering or terrorist financing that involves the use of non-profit organizations (NPOs) as conduits for illicit funds. NPOs may be vulnerable to abuse by criminals who exploit their legitimate activities, reputation, and access to funds and resources. A transaction monitoring process that would alert a bank for this red flag activity is to analyze the source, destination, frequency, and purpose of the funds transferred to and from the charity account, and to compare them with the expected profile and activity of the NPO. If the bank detects any inconsistency, discrepancy, or deviation from the norm, it should raise an alert and conduct further investigation.
References:
* FATF Report: Risk of Terrorist Abuse in Non-Profit Organisations, page 9
* Transaction Monitoring Processes for Red Flag Activity in Banking
* AML Red Flags - What are the Top 10 Indicators? - ComplyAdvantage, section "Red flags related to the nature of transactions"