Insurers can hire private investigators to do sub rosa surveillance in order to deny a claim for long-term disability, or terminate benefits after a claim has been approved. The surveillance usually takes the form of following an applicant around for several days and filming their activities while they are out-and-about on foot, driving a car, shopping,, or performing other routine activities.
The footage that is obtained is then reviewed to see if there are gross inconsistencies between the functional limitations that an applicant has reported and what they see an applicant being able to do on a surveillance video.
For example, if an applicant for disability benefits claims that he has a bad back and can only drive a car for 15 minutes, or cannot lift more than 10 lbs, but there is a surveillance video that shows the applicant driving for more than 1 hour or carrying heavy grocery bags, then those discrepancies may call into question the credibility of the claimant and the authenticity of the disability claim.
It is often the case that the surveillance video does not directly prove an applicant is able to perform his job or perform full time work, but it may raise a red flag to the insurance carrier to deny or terminate benefits.
Then it becomes necessary for an applicant's lawyer to carefully review the surveillance video in order to try to refute the surveillance evidence and rehabilitate the client. Usually, this can be done, but not always.
It's best to try to avoid the surveillance problem in the first place by assuming that someone is always watching when you are out in public place.