When is a CAPTCHA challenge raised?

[semdev]

Our application and I am sure a lot of other peoples applications do a
lot of api calls in the background.

This is how things work with V13.
The email, password, token etc.. are stored in a db and the
application uses that to communicate with the API.

One of the codes that the 2009 ClientLogin API returns is a CAPTCHA.
We have a few hundred accounts being processed in the background daily
and we cannot expect the user to be around when an operation takes
place. The browser is not even around. All the execution happens in a
linux box.
We have no way of alerting the user to a CAPTCHA challenge.

I would really appreciate it if you could provide some details of this
CAPTCHA challenge.

Essentially, what will cause this challenge to be raised?
Is there any frequency associated with it i.e you raise it every few
months or so?
Can we indicate to the API to just return an BadAuthentication code
instead or a captcha challenge?

Supposing I store the Auth Token and after a week when I try to call
the CampaignService api using this token it fails.
I would then make a ClientLogin request and try to get a new
authtoken. Will this cause a CAPTCHA challenge?

Again it would be great if we can indicate to the API not to raise the
CAPTCHA challenge. It doesn't really integrate very well with
applications that run in the background.

[AdWords API Advisor]

Hi,

In general CAPTCHA challenges are raised when there have been
excessive logins in a short period of time, or if there are other
signs of fraudulent behavior. Developers commonly receive a CAPTCHA
challenge if they are generating a new authToken for every API request
instead of reusing the authToken. I don't think you can expect to
receive these challenges under normal circumstances, but you should
provide some handling for them. In your case it may be enough to log
the error and send out an alert to the account owner advising them to
unlock the account.

Best,
- Eric Koleda, AdWords API Team