I hear this term a lot in my line of work. It can be quite astounding to hear at first but then when you think about it, it makes perfect sense. Professionals are the people who do this sort of thing for a living just as with any other occupation.
These people often have multiple claims with different companies for various reasons. They collect a check from each one monthly and still work or do whatever they want. I've caught a few of them doing exactly what they told the insurance company they were unable to do.
How do they get away with it you ask? Well, many lie about their addresses. I've been sent out to addresses where there was no house (just green grass) and I've seen drivers licenses with addresses that don't exist. They used to, before 911 upgrades but now are no longer valid. PIC's exploit this kind of flaw. DMV doesn't yet know the address isn't good or hasn't removed it from the computers. Or possibly the claimant has some piece of residential proof and DMV doens't check any further.
Some flat out refuse to provide an address. They'll only give a P.O. Box and then the insurance company has to work to find them. It's not usually easy but it's often quite fascinating to me when I get to do the research myself.
There's always a way to find them without violating any laws or invading their personal rights. Any activity done in the public view is open to be recorded. So in order to avoid detection, many PIC's won't come outside until after they know we're gone. Some have their friends and family watching the neighborhood and they have sent people to block investigators while they make their escape. So it's a delicate game but PI's have their methods of finding the individual.
But there's a fine line in ending their fraud. You see, the insurance company can't just catch them doing an activity they said they couldn't do once. The courts will look at it as well, anyone would pick up their kid regardless of the pain or maybe the work was urgent and they had to suffer through it once. So what they do is find the person doing the deed multiple times or repetitively for several hours. Only then is it truly useful in court should it be needed.
That's difficult to obtain. These people also use evasive tactics to aid them in discovering us should we be present and once they know we're there they get even more evasive. I've seen people zigzag through each city block and go in circles and take the craziest routes to avoid being followed.