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From Starwood Lurker team :
Please visit  info.starwoodhotels.com  for more information about this incident, available resources and steps you can take.

Marriott has announced a massive breach of data belonging to 500 million guests who stayed at hotel brands including W, Sheraton, and Westin.
Marriott announced on Friday that it had "taken measures to investigate and address a data security incident" that stemmed from its Starwood guest authorization database.
The company said it believes that around 500 million people's information was accessed, including an unspecified number who had their credit card details taken. It affects customers who made bookings on or before September 10, 2018.

http://uk.businessinsider.com/marriott-data-breach-500-million-guests-affected-2018-11?r=US&IR=T
https://www.prnewswire.com/news-releases/marriott-announces-starwood-guest-reservation-database-security-incident-300758155.html

You can enroll in the "identity" monitoring service provided by Marriott due to this breach here, it cannot be called "credit monitoring" because it doesn't provide access to viewing credit bureau report data (as held by Equifax, TransUnion, Experian) nor notifications when credit report data changes :
https://answers.kroll.com/us/index.html