Can lead-acid batteries release CO? Or can a CO sensor detect gasses other than CO?
You're probably picking up hydrogen gas, which is produced when lead-acid batteries are overcharged at high charging voltages (a danger in its own right). This article details a situation similar to yours: charging a lead acid battery in a golf cart (in a confined space) sets off a $\ce{CO}$ alarm, and typical sensors are activated by $\ce{CO}$ at levels of 150 ppm for 30 minutes and/or 300 ppm $\ce{H2}$ for 30 minutes. Because $\ce{CO}$ is not produced by battery charging, the offending gas is thus identified as hydrogen. After venting, the levels recede.
Also see this post from Hawk Environmental, which details how hydrogen gas can create false positives for carbon monoxide. This post goes into a little more detail about the hydrogen/carbon monoxide sensor issue, and the author states that:

Lead-acid batteries produce Hydrogen when charging.
Carbon Monoxide detectors use something called a &quot;Metal Oxide Semiconductor (MOS)&quot; sensor, which detects a variety of gases including Hydrogen.
A MOS sensor calibrated for CO will give a false positive in the presence of Hydrogen gas at ~10% of the actual value. So, that ~85ppm reported of CO was in reality ~850ppm of Hydrogen gas.

Finally, in an article in the September 1996 issue of Fire Engineering, entitled &quot;Other gases may set off $\ce{CO}$ detectors&quot; (here), it is stated that

CO detectors are electrochemical sensors, which detect a variety of specific gases. In most cases, the gas of interest reacts with the chemical solution in the sensing electrode to produce ions (charged particles) that move through the solution to an electron-accepting electrode, completing an electric circuit. The number of ions produced and the magnitude of the current developed are proportional to the concentration of the gas being measured.

and that

Interfering gases of similar molecular size and chemical reactivity may produce false positive response. Major interferents that can affect $\ce{CO}$ detectors are acetylene, dimethyl sulfide, ethyl alcohol, ethylene, hydrogen cyanide, hydrogen sulfide, isopropyl alcohol, mercaptan, methyl alcohol, propane, nitrogen dioxide, and sulfur dioxide.

I suggest you confirm/deny the above by contacting Fluke and asking them specifically if the sensor does not detect hydrogen gas at all and, if so, at what level. I strongly suspect that the well-documented behavior I've found online and summarized here is the cause of what you have observed in this case.