It may have been more obvious if it had been a large round/oval shape instead, which I think is much more representative of a mirror. The test we used for deciding representative members of a category in cognitive science was, If you tell a small child to draw it, what will they draw? I think if you told a kid to draw a mirror, it would be oval instead of rectangular.
I haven't seen an actual oval mirror in years and years, and I see lots of rectangular ones -- but I'm well aware that the canonical form of something in a society doesn't always match what's most common. And my personal sample is not representative, and my attention may be uneven.
Also, while I have seen oval framed art, it's vanishingly rare, which might help with discrimination.