Wiktionary
drop in
n. 1 One who arrives unannounced or without an appointment. 2 (context surfing English) the act of dropping in (see verb below). Often hyphenated '''drop-in'''. vb. (context idiomatic English) To arrive unannounced or with little or no warning; also, to visit without an appointment.
WordNet
Usage examples of "drop in".
Typically, they are caught by an unforeseen change of conditions or combination of changes--a sudden drop in temperature, a cold pelting rain, the realization that they are lost--for which they are emotionally or physically underequipped.
I can't take the time this year, so I thought he'd arranged for Santa to drop in.
Summerset mentioned you were down here, so I thought I'd just drop in on you.