Cryptojacking is the likely cause in this scenario. It involves malware that hijacks the resources of infected computers to mine cryptocurrency, usually without the user's knowledge. This type of attack doesn't typically degrade performance significantly or result in obvious system failures, which matches the situation described, where the machines showed no signs of degraded performance or excessive failed logins.
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CompTIA Security+ SY0-701 Course Content: Cryptojacking is covered under types of malware attacks, highlighting its stealthy nature and impact on infected systems.

A training curriculum plan for a security awareness program should address the following factors:
* The threat vectors based on the industry in which the organization operates. This will help the employees to understand the specific risks and challenges that their organization faces, and how to protect themselves and the organization from cyberattacks. For example, a healthcare organization may face different threat vectors than a financial organization, such as ransomware, data breaches, or medical device hacking1.
* The cadence and duration of training events. This will help the employees to retain the information and skills they learn, and to keep up with the changing security landscape. The training events should be frequent enough to reinforce the key concepts and behaviors, but not too long or too short to lose the attention or interest of the employees. For example, a security awareness program may include monthly newsletters, quarterly webinars, annual workshops, or periodic quizzes2.
References: CompTIA Security+ Study Guide: Exam SY0-701, 9th Edition, Chapter 2, page 34; CompTIA Security+ Certification Kit: Exam SY0-701, 7th Edition, Chapter 2, page 55.

A honeypot is a security mechanism set up to attract and detect potential attackers by simulating vulnerable assets. By hosting a part of the infrastructure online with known vulnerabilities that appear to be company assets, the company can observe and analyze the behavior of attackers conducting reconnaissance. This approach allows the company to get alerts and gather intelligence on potential threats.
Reference = CompTIA Security+ SY0-701 study materials, particularly on threat detection techniques such as honeypots.

A statement of work (SOW) is a document that defines the scope, objectives, deliverables, timeline, and costs of a project or service. It typically includes an estimate of the number of hours required to complete the engagement, as well as the roles and responsibilities of the parties involved. A SOW is often used for penetration testing projects to ensure that both the client and the vendor have a clear and mutual understanding of what is expected and how the work will be performed. A business partnership agreement (BPA), a service level agreement (SLA), and a non-disclosure agreement (NDA) are different types of contracts that may be related to a penetration testing project, but they do not include an estimate of the number of hours required to complete the engagement. Reference: CompTIA Security+ Study Guide: Exam SY0-701, 9th Edition, page 492; What to Look For in a Penetration Testing Statement of Work?