Ok here is a new one on me. I have a 99 Expy with 260k miles. I went fishing this weekend and to avoid draining my battery with the doors open I disconnected the positive cable. Well when I was ready to leave I reconnected the battery and headed to another location for a few hours. Went to leave there to head home and when I open a door a hear a relay kicking on and off. Turns out it was the interior lights and running board lights were flashing. Well the battery is dead and I get a jump and head home. This morning I went to clean the truck out and I hear the relay again. Try to start the truck and NADA. Now here is the weird part. I check the water level and find the cell nearest the negative pole is dry then did alittle bit more inspection and the top of the batter around the negative terminal is concave with a few holes melted in it around the terminal. The battery is a Dura Last battery about 3 yrs olds.

Anyone have any Ideas why this happened?