Thank you very much for responding to me and attaching the link.
I looked at the thread thoroughly. I'm not 100% sure if the answer to my first question is yes or no.
Tozor said that "from looking at the stack trace, it looks like the Adwords client library is actually reloading the auth token". So, the answer to my first question is yes - the AdWordsUser is caching the token. However, later he says "we are caching the AdwordsUser instance in our code (which contains the token)". So, it seems like, if he has to cache the AdWordsUser instance, then it is not done automatically by the library, so the answer to my first question is no.
Assuming the answer to my first question is no, my operation is looping through many accounts and running operations on them. Each operation must create a new AdWordsUser because the AdWordsUser instance is tied to an account. Is that also the case for the AuthToken? If each AuthToken is tied to an account, then running operations on multiple accounts, each with its own AuthToken shouldn't generate the CAPTCHA error, right?
Finally, if I was able to create an AdWordsUser instance for each of the accounts and store them for a week, can I just reuse them? Should I serialize the objects to disk? How are others storing the AdWordsUser objects?