I will skip the first two because I don't understand exactly what you're trying to say.
1) Using the expression "to drop by."
A: When will I see you?
B: Just drop by (and see me).
2) Meeting (bumping into) someone at a shop.
A: Why are you here?
B: I was just [passing by]. (I was in the neighborhood.)
The only instance that I can think of for the moment for "drop in" is this situation:
A chief executive enters a conference room unannounced.
A: Hello, everyone, I just decided to drop in (attend the meeting), and see how everyone was doing.
Or, "He dropped in on us, unannounced. (Meaning: He showed up, unexpectedly.)
Hope that helps.