Is an employee on secondment entitled to claim her accrued leave balance after her return to her governmental employer, and would disclosing this bring harm to her colleagues?

You do not have the right to benefit from vacations mistakenly calculated due to employee negligence, because you had already received your full entitlement. Taking these vacations constitutes devouring people's wealth unlawfully, betraying trust, and failing to fulfill the contract and its conditions, as Allah Almighty says: "O you who have believed, fulfill [your] contracts." You should alert the officials if this is a widespread and rampant corruption so that they stop their negligence; otherwise, you should report their matter to higher authorities to correct the errors, as this falls under enjoining good and forbidding evil.