Sounds like your tank cycled without you bothering to mess with it. I would say that's very lucky. As far as the API kit, I use it regularly because I continue to have ammonia issues, even though my tanks were once very well cycled. Problem with API is that some bottles require significantly more drops than others, hence you will run out of the Ammonia bottles and Nitrate bottles long before you run out of the Nitrite and PH bottles. It seriously irritates me that they say it's good for so many "tests" when nothing could be further from the truth, but it's the best test kit around so for every one full test kit I end up purchasing about 3 separate ammonia only test kits for about 1/3 the price - it's still makes it expensive. But again, I test very frequently - especially my ammonia readings so it makes sense that I'll use up the ammonia test first plus each of the two bottles takes 8 drops - one makes 8 BIG drops while the other makes 8 LITTLE drops so I have an excess of one bottle over another. Again, really frustrating.
Glad to see you're going to monitor your water values but, then again, you've obviously done fine without doing it. I think sometimes I get too hung up on my water values but I need to add Prime every 48 hours to ensure that any tank with high ammonia levels converts the ammonia (toxic) to ammonium (non-toxic). So, for my peace of mind it's essential that I monitor my water chemistry every other day. I've used Seachem's Stablility to try to add additional bacteria (no luck) but have had better luck using Tetra's Safe Start Plus after every water change - my ammonia levels have dropped significantly in my tanks and I'm finally seeing ammonia convert to nitrites and nitrites convert to nitrates - so I think I'm nearly cycled again - but I wouldn't be surprised if it doesn't take a number of more additions of bacteria before I'm back to zero ammonia and nitrites every day. I really hope that will be true because I work my butt off to do weekly large water changes and keep the gravel as clean as possible - while others - such as yourself, don't seem to have to do much extra and you have great water. But who said life was fair?