Q: Something is draining the battery

Even though you did disconnect the battery, thus preventing any "external" current flow overnight, you still should check for parasitic current draw from the vehicle’s electrical system, that is a current flow of (roughly) 50 or more milliamps when the car is "off". It’s also possible there is parasitic (unintended) current draw even while the car runs and if it is of a magnitude that the charging system just can’t keep up with that could slowly, or eventually, leave your battery in a discharged state. There could be an intermittent fault in the charging circuit but normally if the alternator was dropping out, the charge/battery warning light would come on and alert you. There are two things to do at once: check the output of the charging system (have a certified mechanic do it) per the Factory Service Manual protocol and, two, take the battery to a professional battery store, have it fully charged, and then have them run a load test on the battery (load tests can’t be done on discharged batteries). If, say, you find that there is nothing wrong with the battery and vehicle charging output is normal, then clearly there is a problem in your car’s electrical system, possibly including unwanted (parasitic) power draw but possibly ALSO including a circumstance where a faulty starter draws too much current, thus rendering it inoperable unless the battery is practically brand new (battery CCA slowly degrades over time and although a new battery will run a weak starter, an "older" battery might not). The point is you could have a "combination" of problems. If you are electing to have a certified mechanic take care of this from top to bottom (perform all the tests and the obviously needed repair) simply request an electrical component malfunction diagnostic and the responding mechanic will get this resolved for you pronto. Please don’t hesitate to inquire if you have additional concerns.