Even with WiFi turned off it is still the biggest battery drainer (S5 Mini) I have a Samsung S5 Mini and today I noticed that even when WiFi is off, it is still the biggest battery drainer (oscillates between 20-23%). I also noticed that yesterday the battery drain was much higher than it usually is.

I have disabled everything that makes use of WiFi, tried rebooting the phone, but It remains the same.

Do you have any idea what the problem might be, and how can I disable WiFi effectively?
Navigate to Settings -> WiFi -> Advanced (under the context menu) and ensure that Scanning always available is not checked. This feature 'Let's Google location service and other apps scan for networks, even when WiFi is off'.

This is most likely the cause of the battery drain. To further reduce WiFi battery use, you could change the Keep WiFi on during sleep option to Only when plugged in or Never. However, this may prevent you from receiving emails, messages and similar notifications while the device is locked.