Also, it may be a VIEWPORT you are seeing. I don't use 2018 so I don't know if it adds a viewport by default. It doesn't in 2017. But viewports are blue. Can you select it and grips appear? Is a viewport listed in the dimensions palette?
ADDED: Actually, in looking at your screenshot it looks as though there are at least 3 "Untitled" viewports. If you didn't created them and don't need them, select "Clear All" in the Viewports area of the Measurements palette.