Thanks for all of the information. After doing some further investigation and monitoring when it goes off, here's what I came up with.
First, it only goes off if I'm charging my batteries with the vent caps off. And yes, I did charge them once overnight with the caps off.
It never goes off if I'm not charging my batteries. Now having said this, I thought Hydrogen gas, which is what is given off when you charge bats, was lighter than air and will naturally rise. My CO detector is under the sink cabinet in the kitchen about 4 feet from the stair well where the bats are.
I guess the morale is to vent the gas away from the kitchen