When work is done across multiple locations with no single location to get the true number of hours worked, the hours will remain unverified. And when there is no centralized way to report these hours, the process is typically seen as a formality rather than a documented fact. The problem here is not that you will be charged for hours not worked; it is that when you cannot prove to your customer how services were delivered, the relationship begins to break down.
In most cases, manual time sheets fall apart as employees go from site to site. It is hard to conduct spot checks, and the manager has to take the employee's information on good faith. This is a commercial risk when you have a contract that could be audited or disputed by your customer.
The gap between scheduled hours and verified hours will always increase over time in any mobile workforce. Any system that relies on an employee's administrative efforts and trust will create operational blind spots that management does not even know exist until they have a compliance issue or a billing dispute.