Turn on Promotions feed, then tick AI promotions feed on the coupons you want to promote. Pro writes google-promotions.xml for Google Merchant promotions, with the code, the discount, minimum spend and dates. Products on a scheduled sale also get their sale date range in the Google feed.
A coupon is left out, with the reason shown on the Profiles and channels tab, when it is expired, has reached its usage limit, is limited to certain customers, uses category rules or exclusions, or is not a percentage or fixed amount discount.
The code becomes public in the promotions feed. Only mark coupons you are happy to share with every shopper.