I got the GPS bracelet I bought online, originally intended to prevent children from getting lost. The bracelet has a red appearance and looks somewhat like a toy, but it could pass as a sophisticated accessory that suits a junior high student's style.
Anyone with knowledge about such gadgets would probably recognize its true purpose at a glance, but from what I've observed, Mazao's understanding of modern electronics is quite superficial. For example, she knows what a refrigerator and a TV are, but she's quite unfamiliar with smartphones. She was curious when she saw the sports bracelet I bought on a whim, asking me what it was. It was then that the idea of gifting the GPS bracelet first occurred to me.
Admittedly, a thermal energy marker is certainly more covert than a GPS bracelet, but since I've started to doubt whether the superpower-based "Firefly" could be discovered by Mazao, I naturally couldn't be completely confident in the superpower-based thermal energy marker either.