I feel the same way; there were a few times I saw things that made me go, "hey, that wasn't in any changelogs..."; the dev team may have a requirement to put everything into the changelog, but it really doesn't seem like that's happening. I'm absolutely positive there are stealth changes being made, but it's not easy to pin down what/when/where unless you make specific notes when you see it.
Take for example, xeno burrowed larva spawn rates. I don't think that's been mentioned anywhere in changelogs, but it's defnitely been getting adjusted every so often. Likewise xenos were absolutely murdering marines for a good 2 weeks straight, and then suddenly that stopped happening. What was changed? I don't know. It's not in the changelogs, but something defnitely changed.