http://stuver.blogspot.com/2011/03/big-dog-in-envelope.html

They occasionally inject fake signals into the detectors to test the detection process:

A blind injection is a test the higher-ups in LIGO can do to make sure that the data analysis methods are doing what they need to be doing. Basically, a very small subset of people in the collaboration (think like 2 or 3 people) inject a fake signal into the detector and this injection is not recorded anywhere like the other injections we regularly do to test things like detector calibration, etc. The fact that a blind injection exists is sealed away (in what we metaphorically call an envelope) until all due diligence is done and the collaboration is ready to declare that the signal is a detection unless it is a blind injection (that it, we prove that it is nothing in the environment or and nothing was wrong with the detectors detectors that caused the signal).