What is the most likely cause of malware alerts detected on employees' workstations after returning from a trade show?

The most reasonable explanation for the malware alerts detected on employees' workstations after returning from a trade show is that a USB flash drive attempting to run malicious code is the most likely cause. During trade shows, employees often exchange information through USB drives, which can introduce malware into an organization's network.

The scenario suggests that upon returning, several workstations exhibited alerts, which aligns with the common practice of using USB devices to share data and materials from the trade show. If a USB flash drive contained malware, once plugged into corporate workstations, it could execute malicious code, leading to alerts being triggered by the organization's security software.

While a worm propagating through the intranet could explain malware spread in a closed environment, it's less likely to be the immediate cause when employees return from an external event. Similarly, fileless viruses and Trojan horses are feasible threats, but the direct exchange of USB drives at trade shows makes the USB flash drive scenario significantly more plausible in this context.