Looking on the internet deeply has found these results:

I am at Starbucks is the most popular phrase on the web.

I am in Starbucks 157,700 results on the web	I am at Starbucks 178,600 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I decafecated just this morning in Starbucks, all over. So spend some time in Starbucks. I am the manager at the Starbucks where he worked. So every person in Starbucks with a laptop is a published author? So spend some time in Starbucks. No, I am not working at Starbucks. You know, when you are standing in line in Starbucks, 30 percent of transactions in Starbucks on any one day are actually being made with Starbucks Star points. Back there in Starbuck how did my boy do?	He was buying a Frappuccino at Starbucks. The line at Starbucks was atrocious. No, I am not working at Starbucks. It would be nice not to write at Starbucks with the other novelists. I worked at Starbucks all through college. I can't even get a coffee date at Starbucks. They think I work at Starbucks. I am the manager at the Starbucks where he worked.

I am in Starbucks

157,700 results on the web

I am at Starbucks

178,600 results on the web

More popular!

I decafecated just this morning in Starbucks, all over.
So spend some time in Starbucks.
I am the manager at the Starbucks where he worked.
So every person in Starbucks with a laptop is a published author?
So spend some time in Starbucks.
No, I am not working at Starbucks.
You know, when you are standing in line in Starbucks, 30 percent of transactions in Starbucks on any one day are actually being made with Starbucks Star points.
Back there in Starbuck how did my boy do?