It's true that you can't know for sure without reviewing a medical record, a payer can't know for sure whether or not the unbundling was appropriate or not, but there are a lot of clues that allow payers to identify likely errors, so I would actually disagree a bit with the post above. Having worked for data-mining audit contactors for several payers, I think it is actually quite easy for payers to identify bundling errors.
I'll give you just two examples to illustrate this: 1) if a provider does an excision of a skin lesion and performs a simple closure, the closure can be unbundled with a modifier. But if there a no other services on the claim, and the only diagnosis submitted is for the skin lesion - no lacerations or other problems requiring a suture - then this is almost certainly an error, otherwise what is the provider closing other than their own excision? 2) for certain arthroscopic procedures, debridement is considered bundled unless done on the contralateral joint, and some providers can unbundle the debridement on the same joint with a modifier - again this is easy to identify a suspect claim when all of the diagnosis codes submitted are for only the left or only the right side.
All that aside, I'd answer your original question by saying that I think that unbundling errors by providers are extremely widespread. In my experience, there are so many errors that payers simply can't go after all of them, and just have to focus on those which are most cost-effective to target. When I did reviews of large volumes of payer claims, I was just stunned by how many obvious errors there were out there throughout the industry. I saw many providers who just appended 59 modifiers to almost everything they submitted just to ensure it got paid. And as payers recognize this, we're seeing now that many are creating policies where they are putting the burden back on providers by just deny claims up front without any record review and requiring provider to appeal in order to justify the unbundling (as with the example mentioned in the post above with modifier 25). It's understandable that providers are upset with payers, but it's unfortunate that many do this because it hurts the providers who are conscientious and trying to do the right thing, and in the long run it also ends up hurting everyone in that it drives up the costs of care.