. . . having a lot of unused credit can be seen as a warning sign to lenders. . .
Well, my own financial education is a bit more than 'google-born' and I don't particularly agree with your statement.
Years ago, lenders would have more discretion in making loan decisions. These days it is clear that credit score is the largest factor, plus of course income and ability to repay based upon other debts and obligations. Discretion has largely evaporated, unless perhaps you are in a high income category and able to access personal banking services that most people cannot qualify for.
Credit score information I refer to from time to time, on my bank site and so forth, always reports credit usage (as a percentage of total available credit) and states that lower is better. Most people who have some number of credit cards will see their total available credit increase from year to year. Your comment suggests that this could be a bad thing. I don't think that is generally true.
I also think that credit card issuers will move fairly quickly to reduce credit limits on cards, if a card holder starts to accumulate balances on their cards and begin to use a larger percentage of their available credit. They will also sometimes lower limits if card utilization is low. These are my own observations after many years of personal experience.