You get offered a job away from your home. Your travel to it. You work there under a year. You come home. You take a job some place else. The traveling tech's life in a nutshell and I hope it is that simple. You do not travel looking for trouble. Your goal is to legally earn as much as possible and keep of much of it as you can. Nobody is looking to be dishonest. We just want to follow the rules, and stay out of trouble. I have always been told you do NOT have to report left over per diem or travel pay. If you can save some of this money due to better money management practices, why should you not be able to keep it? Some of the posts put that in question. If everything I say in this paragraph is a correct assumption, being honest and following the law are the same thing. If there is some sort of trap in what I am describing, only then I have a concern. I no longer travel, my daughters do and always want then to follow the law and be honest at the same time.
Honesty and following the law are not necessarily the same thing, but being honest makes it easier to stay within the law.
If your activities are as you describe them, no problem.
Unsubstantiated reimbursements have to be reported as income. If you receive a per diem allowance, and that allowance is at or below the GSA rate, then the allowance itself is substantiation of the expense. If the GSA rate is $123, and you get $90 per day (under an accountable plan), you do not need receipts to show that you had $90/day in expenses. You do not need to return or report any incidental excess that may be left over from that $90.
The problems arise when the person who is in the situation above tries to deduct the $33 difference as an expense (Lots of people do it. They are breaking the law.) Also, it is trouble when the person has worked at the same site year after year, never working at any other place for nearly as much time, probably owns a house there or has a lease on one, perhaps brings the wife and kids along, yet still collects per diem there.
Your assertion that nobody is looking to be dishonest, and that "we" just want to follow the rules may be true about you and some of the people you know, but it is certainly not true of all the traveling nukeworkers. The industry is infested with people who use non-existent technicalities within the law to justify improper payments of per diem or illegal tax deductions. That is not the same as keeping the few measly bucks left over from your properly paid per diem allowance. They are going to get caught; they are going to ruin everything for all the honest people, and before long we'll all be submitting expense reports with receipts for every meal. We'll be going through a pile of paperwork every week just to get reimbursed penny-for-penny for the expenses we can prove. There won't be any excess to bank, and it won't be worth the headache to keep track of every little expense.
It is already happening. That is the point of this whole thread. A few scammers have called attention to per diem payments to everyone. They have everyone under the microscope for the deeds of a few. Every time this happens, everybody pays the price.