Standby mode still draws power even when you're not using the device. The power draw may come from your other wireless services, such as mobile data, NFC, and location services.
Another reason for power draw may be a recent update, as devices relearn your usage habits after every update, which causes devices to heat up and draw more power even during standby.
If the problem persists, consider running a diagnostics test via the Samsung Members app, and then bring the device to a Samsung Service Center for repairs, if needed.
