I had planned on replacing my battery next month. I've owned the car for a little over three years now and figured it was safe to assume that it had been in there a bit before I bought her.
Yesterday I went to start her after she had sat for a few days and she cranked noticeably weaker than usual.
Not being one to push my luck, I moved up my planned replacement and made a quick run to Advanced Auto for a battery.
This morning I headed out to the garage to switch them out. I pulled the old battery out and found to my surprise that the PO never hooked up the vent hose. The hose was laying off to the side of the battery and the elbow connector was still inside the sealed plastic pouch still taped to the side of the 3+ year old battery.
The new battery is now properly installed. Can't make this stuff up.