The most valuable witness in modern investigations is the one you don’t actually see.
Whether it’s doorbell cameras or smartphones, even smart speakers and connected appliances, the devices we live with every day are constantly collecting data without any active input and can provide vital intel in modern investigations. Passive data collection is the term used to refer to information being captured in the background, whether that’s your location, audio or video recordings.
For forensic investigators, passive data is now one of the most powerful and decisive sources of evidence. With eyewitness testimony often unreliable, passive data has largely removed the archaic dependence on unreliable witness testimonies and has changed not only how cases are solved, but how investigators think about where evidence lives in the first place.
Always on
Modern smart devices are always on. Microphones listen for wake words, cameras monitor for motion, smartphones continuously log locations, app usage and system activity. Even those we rarely think about, such as thermostats or other home appliances like refrigerators, quietly record patterns of behavior.
These devices have the ability to collect location, sound and video even if you don’t expect or initiate it. Home security systems are a prime example. Without any subscription, many cameras will detect and register events, such as a person approaching a property or a vehicle passing by and will store the data either locally on the device or transmitted to backend servers managed by service providers such as Google or Ring.
This distinction between local storage and backend storage is absolutely critical. Some data exists temporarily on a device, while other data is retained remotely, sometimes for far longer than users realize. The backend data is what becomes the most valuable to investigators as it can persist beyond what is visible to the device owner.
The imperative role of smartphones as a source of passive data cannot be ignored. Every application on someone’s smartphone, from social media platforms to navigation tools, request access to core sensors on the device such as the camera, microphone and location, which means behavioral data is constantly and continuously being generated, unless disabled by the user.
The crime screen
Passive data offers a uniquely powerful element, an objective, time-stamped record of events that doesn’t rely on human memory. In practice, this can be the differentiating factor between a stalled case and a solved one.
In the tragic Nancy Gutherie kidnapping case, images and event data captured by home security systems, collected without any deliberate action, provided crucial investigative leads, without which there would have been very little to go on. Investigators were able to pull data stored on backend servers, which revealed a masked man on her doorstep.
However, the retrieval of passive data is often misunderstood, as it is not simply ‘stored in transit’ or fleeting, but is frequently retained on backend servers and can be recovered by those with expertise and authority to access it. This is where investigative skill becomes essential.
Knowing that passive data exists is merely a starting point. What investigators need to understand is where different devices store data (locally versus cloud), how applications structure and change that data and how to identify relevant artifacts across evolving systems. If investigators don’t understand these nuances and can’t identify them, passive data is rendered useless.
The challenge is compounded for investigators due to the fact that applications and software constantly change. File formats continue to evolve, storage locations shift and data structures are updated. There is no “how to” guide on how this data is stored so it’s up to digital investigation professionals to stay up-to-date on training so they know where to look and what to make of the information.
Even small changes in these infrastructures can mean tools miss critical evidence entirely. Strong foundational skills, particularly in areas like database analysis, remain essential. Because of this, passive data is a resource and an opportunity for investigators, one which can allow them to uncover information that may not be visible through conventional analysis and go beyond ‘pressing a button’ to retrieve evidence.
Passive data, active fear
Despite its investigative value, passive data collection sits at the center of ongoing debates around public safety and privacy.
The thought of devices constantly listening, watching and recording our behavior is largely unsettling for most people which, to an extent, is justified. These technologies are powerful and when misused, the consequences can be huge. Yet there is a gap between perception and reality. If someone truly wants to avoid passive data collection, the only real option is to step away from modern technology entirely. This means not having a smartphone, a single smart device, no connected systems and, for most people, this is not only impractical, but impossible.
The same systems that enable targeted advertising and personalized services are the same ones which can provide evidence in criminal investigations, protect homeowners from liability, help locate missing persons and capture critical moments in emergencies.
The best way to think of this is to see passive data as akin to insurance. When paying for insurance, it isn’t with the intention that it will impact our lives on a daily basis. Rather than seeing passive data as unsettling, consider reframing this data as ‘forensic insurance.’ In the event a crime should ever take place, knowing multiple people can access one’s location or see one’s last point of contact can make a world of difference to bring about a resolution.
Silent witness with the loudest impact
The challenge of passive data collection is not to eliminate it but to use it responsibly. This demands clear legal frameworks, strong ethical standards, greater transparency from technology providers and better public education.
At the same time, digital forensics is a constantly evolving field. Applications change, devices update and storage formats shift, what works today may not work tomorrow. Success depends on adaptability, continuous research and collaboration.
Passive data does not replace traditional investigative methods, it strengthens it. As a silent witness, the data can reveal critical details which can completely transform a case, if investigators know where, and how, to look for it.
About the Author
Heather Barnhart is the Senior Director of Forensic Research at Cellebrite and a SANS Institute fellow. She advises on strategic digital intelligence operations and educates both the public and industry professionals on the latest challenges in the space and how Cellebrite helps address them. For more than 23 years, Heather has worked on high-profile cases, investigating everything from child exploitation to Osama Bin Laden’s digital media. She has helped law enforcement, eDiscovery firms and the federal government extract and manually decode artifacts used in solving investigations around the world.