Passive OT Monitoring and Continuous Asset Discovery

Operational technology (OT) environments are not static. Devices are added, firmware is updated, configurations drift, and network behaviour changes as processes evolve. In real OT environments, these changes often occur incrementally and without a single clear point of transition. Passive OT monitoring combined with continuous asset discovery is designed to reflect that reality by maintaining an up-to-date view of what is present and how it behaves, without interfering with operations.

In this article, we look at what continuous discovery means in practice, how changes are detected over time, and why one-off discovery exercises quickly lose value in real OT environments.

What passive OT monitoring means

Passive OT monitoring observes network traffic without sending probes, scans, or test packets into the environment. It listens to communications that already exist between controllers, HMIs, servers, historians, and other OT components.

Because it does not interact directly with devices, passive monitoring is generally well suited to environments where stability, uptime, and vendor support constraints limit the use of active scanning techniques. These constraints are commonly present in legacy or safety-critical systems, where even low-impact scanning can be difficult to justify operationally.

From this observed traffic, monitoring tools can infer:

• which devices are present
• how they communicate
• which protocols are in use
• how frequently assets interact

This forms the basis for continuous asset discovery.

What continuous discovery means in practice

Continuous asset discovery is not a single inventory exercise. It is an ongoing process where asset information is updated as the environment changes.

In practice, this means:

• assets are identified as they appear on the network, not just during a scheduled scan
• existing assets are updated when their behaviour changes
• dormant or decommissioned devices are flagged when communications stop
• relationships between assets are refined over time as more traffic is observed

The asset inventory becomes a living record rather than a static snapshot. In many operational environments, documentation lags behind reality, particularly where systems have evolved over long periods or multiple ownership phases.

Detecting changes over time

One of the main advantages of continuous discovery is the ability to detect change, rather than simply list assets.

Passive monitoring can highlight:

• new devices joining the network
• changes in communication paths between existing assets
• protocol changes or new services being used
• unusual increases or drops in traffic volumes
• firmware or configuration changes that alter network behaviour

These changes are often subtle and may not trigger alarms at the operational level. In practice, it is common for such changes to be noticed only after secondary effects appear, such as reliability issues or unexpected dependencies. Continuous monitoring provides historical context, making it easier to understand when and how changes occurred.

Why one-off discovery quickly becomes outdated

A one-off asset discovery exercise can provide a useful baseline, but its accuracy degrades quickly in most operational environments.

Common reasons include:

• temporary engineering work that introduces new devices
• replacement of failed equipment with different models
• software updates that change how systems communicate
• undocumented changes made during incident response or maintenance
• gradual network expansion as processes evolve

Without ongoing visibility, these changes accumulate silently. Over time, asset inventories no longer reflect reality, which can undermine risk assessments, incident response, and compliance activities that rely on accurate information.

Operational benefits of ongoing monitoring

Continuous passive monitoring supports day-to-day operational awareness, not just security objectives.

Key operational benefits include:

• improved confidence in asset inventories used for maintenance and planning
• faster identification of unexpected changes that may affect reliability
• better understanding of system dependencies before making changes
• historical evidence to support troubleshooting and root cause analysis
• reduced reliance on manual documentation updates

By aligning asset visibility with how OT environments actually change over time, continuous discovery supports more informed decision-making without disrupting operations.

To summarise

Passive OT monitoring and continuous asset discovery recognise that OT environments are dynamic, even when they appear stable. Maintaining visibility through observation, rather than periodic intervention, allows asset information to stay relevant as systems evolve.

Used appropriately, continuous discovery provides a clearer operational picture while respecting the constraints and sensitivities that define real-world industrial environments.

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