Testing the location of a backdrop

Tests like "if is in " don’t work how you would expect, I think because of the way backdrops are implemented as single-location objects that move around as necessary. A small example:

[rant]Foo is a room. East is Zoo.
The blimp is a backdrop. It is in Foo and Zoo.
Every turn, say “blimp in Foo: [whether or not the blimp is in Foo].”
Test me with “z / e”.[/rant]
Is this explained in the documentation anywhere? If not I’d like to report this as a bug, since I think someone might reasonably want to test whether a backdrop is in a particular room (you can imagine a case where the backdrop is placed into rooms satisfying some complicated description D, so that it would be much simpler and clearer to write “if B is in R” instead of “if R matches D”).

The only relevant thing I could find is WI 3.25’s statement “a backdrop has no location”, which incidentally is not true (the location evaluates to a room unless the backdrop is nowhere, but which room you get is arbitrary [i.e. it depends on the objectloop order]).

The caveat is mentioned in example 81:

Moving the information to the documentation’s main text is technically a feature request, so you could post to inform7.uservoice.com.