Trying to Find one word for a phrase "Improving without realising"

I’m writing an email to colleagues explain progress over a project, and I’m breaking it down into small phases that we went through. For one of the phases I’m trying to find a word that best describes "Improving without realising/noticing". As a team of work has been going on and we just never noticed how far we’ve come. Need help finding a word for that.

TIA.