There are said to be three conditions which must be met before you can get a court to force reimbursement based on "unjust enrichment": an actual enrichment or benefit to the defendant, a corresponding deprivation to the plaintiff, and the absence of a legal reason for the defendant’s enrichment.
For example (and only theoretically as many countries have laws which have modified equity law in some situations), if you found somebody else’s cash and spent it, you might be sued for reimbursement under unjust enrichment.
The legal theory behind unjust enrichment is the constructive trust, which the court imposes upon the circumstances to hold the person unjustly enriched as the trustee for the person who should properly get the property back, held to be the beneficiary of the constructive trust.