Do not cross the imaginary line

I should have taken a picture of that moment. I was at the museum, looking at some artist I had never seen before. Two sculptures made out of glass were side by side. Their design was intricate and had some sections that invited to look inside the structures, hence, get closer to them. I clearly saw there was a marked line on the floor in front of them but nothing on the sides which suggested you could walk behind them. I was doing just that when I heard a lady from the museum say I could not. I walked back and apologized but told her there were no markings on the floor but she quickly replied (and I remember her somber face vividly): "You can´t cross the imaginary line".
Sheesh, what was I thinking of? Of course you can´t. Hehe.