iPhone Hotspot: who is connected?
Other devices (including iPads, iPods, Macs, and other iPhones) connected to the same iCloud account as your iPhone will, if in WiFi range, automatically connect to your iPhone's hotspot when it's turned on (according to this Apple Support page and my personal experience). That means if any of your other devices are lying around, they will automatically connect to your iPhone's hotspot using iCloud.

tl;dr: Other devices tied to the same iCloud account might automatically be joining your iPhone's hotspot network.