It only appears when the event location is entered for an event which lives in a Google calendar. (I have a Google CalDAV account set up in my Calendar.app) If I make the event go to a local or iCloud calendar, the icon disappears.
The icon appears as soon the field loses focus and the location text becomes "tokenized" (like someone’s name and email in Mail.app for example). It is a tiny exclamation point inside a yellow circle. The tooltip just repeats the location text when hovering over the icon
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Best Answer
You can find what the exclamation point means in Calendar Help>View events>Calendar symbols:
The explanation "the item isn’t recognized as a valid entry by the server" is somewhat misleading. Even when the event is created using Google's web interface (so you would expect the server validated your location information) Calendar complains about the location and shows the dreaded exclamation point.
There seems to be some kind of mismatch between the information Google calendar exports through CalDAV and what Calendar expects. This is probably also the reason why location information in an imported Google calendar is shown inside a bubble:
That's not the case for iCloud Calendar events.
A Google search shows many posts of other people who have the same problem. Nobody seems to have found a solution, though. I don't think you can do much about it.