Answer: There are many reasons for this. Most revolve around skimping by the vendor — on everything from insurance to training to instrumentation.
Let's look at one representative example, thermal imaging...
Most TSCM providers these days offer thermal imaging as a detection technique. The skimpers use ineffective, cheap cameras – just so they can claim this capability. It is a dishonest marketing ploy which lets skimpers "say" they are in the game.
Cost:
• Outdated and low-end utility thermal cameras are available on ebay for less than $2,000.
• High-sensitivity / resolution thermal cameras cost between $25,000-$50,000.
DIY Test:
A TSCM-capable infrared thermal camera will clearly show heat from a fingerprint after an object has been lightly and briefly, touched.
Generally speaking, low-cost equals low probability of detection. Effective TSCM service costs are driven by capital / educational investment... and sincere commitment.
Moral: A cheap sweep is worse than no sweep. Bugs aren't eliminated, just your sense of caution, and budget.
[sotto voce] If you like cartoons, hire a clown.