What finance fraud red flags do you see very often in the execution of project?

Key here is what you consider as a "fraud". In fact, if you are inside a company, what matters is what the company consider as a fraud. A company will have a governance process in place to detect anything related to fraud using tools like AI to detect it. But at the end is up to each person who works in the project.

I do agree with Kiron.
Moreover, I see some people have suppliers to charge/quote based on the budget and share the extra.

Sergio makes a good point. One thing I see as questionable is billing overhead to a client's project. The company may have rules such as if you are interrupted for less than 20 minutes, for something non-project related, that time still gets billed to the project. I might disagree with that since you can hide a lot of overhead as billable time that way, but it is still the rule and I'm not a legal expert.

Fraud is taking or getting something that is not yours through devious means. I interpret fraud as a criminal action rather than mismanagement leading to wasted resources. Dictionary definition: - "wrongful or criminal deception intended to result in financial or personal gain".

When fraud is suspected within an organisation, which may be identified through audits and other governance processes, it can be investigated internally and, if not significant, resolved accordingly (at a senior level).

However when fraud is suspected of external parties - suppliers and such - as may be determined though contract reviews and billing analysis, then it invariably needs to involve external authorities (police). The problem may be much deeper than project impact.

+ for Kirion answers.

I am always suspicious if the estimate for completion is not changing for months. Then one day the EAC is turning above your cost budget + risk buffer. Which makes risk buffer management completely obsolete.

Using more expensive resources than planned - just because they are on a bench but keeping the same cost rates. The list goes on.

But reporting pre-agreed time of resources is, for sure most popular.

In a former position I have seen costs allocated to a project before the physical items were allocated from inventory to the job (machinery-based project). This was a huge red flag for me and in retrospect it should not have happened that way, I should have been more affirmative in expressing my concerns that the costs should not be allocated until they items are moved to WIP.