Contact tracing is pretty common. Four people, all with the same symptoms, go to the ER. All reported eating the same ice cream brand and flavor that day. Hospital staff are able to put this stuff together pretty easily.

As far as how the ice cream got there... it's pretty simple, albeit so stupid. About 6 months ago, the owner of the ice cream shop made weed infused ice cream for personal use and to share with friends. Didn't label it. Stuck it in the freezer. Completely forgot about it. A staffer of Roots took it out of the freezer, having no idea, and served it.