I agree that sounds unlikely. I have lost a lot more in a 20 gallon that is younger than that and there was no ammonia to be found.Thanks
If you mean the the API multi test as in the AquaSpin tester, that's the same as what mine used, it showed my tank had 0.1ppm ammonia despite it being a year old. I did recently lose a large turbo snail, which in a 20 gallon could add a lot of nutrients but I am unsure if that'll cause ammonia, since a year old tank with lots of rock and filtration should process that
Anything that uses the API chemistries is suspect in my book.