x1 and x2 are indistinguishable from each other, they seem to share all their properties except maybe their location and identity, as perceived by observer x3
This is not mitmlu, which means "x1 and x2 seem to be the same individual(s)". This is rather about observable appearance/properties, as in "this box is the same as that one" (they're indistinguishable). See also mludata, mintu.