I see smoke from under the hood and the battery is dying in days

my 1999 S280 benz started to smoke very badly while it was parked an the engine turned off . The ac blows hot air car had to be towed any suggestion/ THANK YOU CAR was running ok for about 2 miles I stopped to make a purchase when I came out it was all smoke we disconnected the battery and had it towed
PRIOR TO THAT it has something which drained the battery 7 every 3 days so we4 have drive and disconnect the battery