International Encyclopedia of Hospitality Management

A walk-in refers to a person who arrives at a lodging facility without a reservation. According to the American Hotel and Motel Association, a walk-in is an excellent opportunity to increase sales and occupancy. Therefore, treat the walk-in guest just as warmly as a guest who has a reservation. First ask the guest the length of his/her stay to ascertain that a room is available for the requested days. If space is available for the entire length of his/her stay then the available rate should be quoted. The walk-in guest is an excellent opportunity to practice upselling and maximizing potential revenue for the property.
A person who says they have a reservation that cannot be found would also be treated as a walk-in. In this case the front desk staff must try to minimize the loss of the reservation by moving on quickly and asking the guest for the necessary information to check him/her into the property. However, if the lodging facility is in a sold-out situation, the clerk should make every effort to find the reservation, and if it cannot be found should apologize and offer to locate the person in alternative accommodation (walking the guest).
American Hotel and Motel Association (1996) Hospitality Skills Training Series: Front Desk Employee Guide. Lansing, MI: Educational Institute of the American Hotel and Motel Association.
Stutts, Alan T. (2001) Hotel and Lodging Management An Introduction. New York: John Wiley & Sons.