Oh geez. I don't know. Maybe he's cooking up meth? Honestly, it doesn't really matter because he's doing something horrible to himself and to you. Are you talking to anyone about this? Are you looking for support?
Hi, some places have substance abuse charities that you can phone and chat anonymously about concerns. They are also good at knowing what people are abusing by the description given so if you have that in your area its worth a try.
Is there anyone else in the household who might have also seen your dad. If there a strong smell then its likely others in the home would notice too.
Is your dad taking other drugs that you know off?