1
general::
Phrase(s): stop by (some place) [and] stop in (some place)
to visit some place, usually briefly. • I was coming home, but I decided to stop by my aunt’s on the way. • She was very glad that I stopped in.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, stop in. Pay a brief visit, as in I hope you'll stop by this afternoon, or He stopped in at Martha's whenever he came to New York on business. The first term dates from about 1900, the variant from the mid-1800s.
American Heritage Idioms