So, she keeps her chastity key with herself, hidden somewhere in your house or appartment that you will never find.
HerOn your wedding night, when you discover her chastity cage and as you give her your key, she gives you hers. You will each be the other's keyholder.
YouSomeone around you (pick on the next step) has her key and his or her own motivation for keeping the key. Boonsri trusts that person blindly.
Someone you both knowFrom time to time, the key will arrive by mail or special courrier company, allowing you to remove Boonsri's device. The next morning, it's always back on and the key has disappeared until the next time it arrives.
A Stranger