I didn't quite get your question. Did you ask for
- Having detected a signal and then losing it.
- Without capability to detect a signal, not due to deliberately ignoring it.
?
Therefore, I am responding with answers to both cases.
Case 1.
1.1
- Retaining the same capability of detecting the signal, but the signal has faded.
- The signal has diminished from our radar. It has disappeared.
- The signal has atrophied.
1.2
- The signal did not fade, but the capability of detection has waned.
- We lost focus of the signal.
- With the introduction of stealth weapons, the capability of radar has diminished.
- Russia is seeking to bolster their declining military capability.
- Our ability to detect alien warships has atrophied, after the Xvorkian Royal Guards enveloped our planet with a shield.
Case 2.
- The equipment was oblivious to the signal.
- We were unaware of the presence of any ghost.
- We were incognizant of the imminent armada of the Japanese imperial navy upon Pearl Harbor.
