Hidden Nav failure
The un-suspected fault with a broken connection to one leg of the Nav antenna would normally show up when testing the system as insensitive operation. Typically you're looking for the fail flag not to appear until you attenuate the test set by at least 55dB, and with a leg disconnected, it normally fails at 30-35dB down. On an EASA aircraft maintained under CAA LAMP, this test is done on every annual, but on most N reg, there is no requirement in the maintenance schedule