it happens all the time since the days of 4.0.4, the lockscreen flicker, there are ROMs out there with patches, and CNA doesn't have the problem, at least with my phone. it's absolutely normal, just a small software glitch, so nothing for you to worry about.
Nexus S i9023
[ROM] Xenon HD
[Kernel] Air Kernel 1000mhz/100mhz SmartAssV2
Nexus 7 CNA