Drop In Meaning

(surfing) the act of dropping in (see verb below). Often hyphenated drop-in.

Example: One who arrives unannounced or without an appointment.
  The office was efficient, but not well equipped to handle drop-ins.
  2005: Drop-ins can and do happen by accident, as well as through frustration and confusion in a crowded lineup. — surfline.com Surfology [1]