You're conflating two different things...maintenance free & venting the battery are TWO DIFFERENT THINGS.
As TX5 stated in his post above, the natural processes of a lead-acid battery will produce off-gases that must be vented to the exterior of the vehicle (especially since the battery is enclosed). Don't be fooled/confused into thinking that a maintenance free/sealed battery doesn't still require to be vented...because it is still producing off gases. There should be a vent hole either on the side or the top center (location of vent hole depends on battery class size & battery mfg).
Look for either a 49/H8 or 95/H9 battery class size. The cost can range from approx $100 - $200+...and many of the batteries are manufactured by the same company and only come with a different colored battery case and label and have the same specs.
(Omodos...this list doesn't apply to you since you're in Cypress...these are USA battery mfg's & brands)
Not venting a lead acid battery when it is enclosed can lead to it exploding if a build up of gas is combined with a spark...and a spark can occur while driving around and the alternator sends an overcharge back to the battery. If you've unlocked your OBC...you can drive around with TEST 9 running and note at times the voltage can spike upwards under certain driving conditions...thus a spark and a build up of gas can cause an explosion like you see in the pic below of a battery that exploded in an e38 7 series from not venting the battery:
Not venting the battery can also cause corrosion as well. Many BMW owners with batteries in the trunk will find that the rear tail light if in the same compartment with a non vented battery can fail due to a build of corrosion on the bulb holder contacts. So again, if you purchase a lead-acid battery (maintenance free or not)...it still produces gases that need to be vented to the exterior of the vehicle. When you replace your battery or have someone else replace your battery...
MAKE SURE THEY HOOK UP THE VENT TUBE!!!