Test Sign Off, n: 1. A promise from the test team that all tests have been run and are passing to the teams satisfaction. (Secretly also means 2. The condition of having assigned blame for all missed bugs to the test team. 3. An impediment to shipping software quickly.)
If your test signoff is all definition #1 you are in good shape. A test sign off doesn't add quality, it only proves a baseline. Injecting quality in your product has to start from the well-spring.