I'm having a hard time accepting that this happened based on your description of two gmail accounts that are only casually linked (by sharing a calendar and emailing each other). If that were the cause, I would expect to see all kinds of weird stuf every time i use Google products, because of my equally-casual connection with several other gmail users. And I definitely dont' see stuff like that.
Were you able to replicate the behavior?
I'd love to see if I can replicate it myself, so asking more detailed questions of your setup inline:
This morning my wife grabbed her phone to do a search for activities at Chelsea Piers. She followed that up with another inquiry for hours at a local mall.
What kind of phone? What app was search done in?
Five minutes later I launched Google Now for stock updates and what do I see?
I assume this was on your phone, not hers, and it's a modern Android. (please correct if I'm wrong) Are you on a shared plan with her phone?
I then followed up with a search. As I began the query with our local town it auto-completed it with mall hours.
What is the small town's name? (just want to try googling it as me and see what the auto-complete is)
Were you still on your phone at this point? If not, what browser were you in and if it was chrome, were you logged into your gmail account?