I've noticed this problem too, and it seems to happen when I've temporarily been without service (e.g., underground parking garage). When I come back to the surface, it looks like the phone still doesn't see the signal for awhile, unless I wake up the Turbo manually. In a day, if I'm in my parking garage 2x for a period of <5 mins total, I still end up with ~8-10% "time without signal" by the end of the day. I've figured this out just by seeing that the two periods of 'red' signal are around the times that I would've been underground.
I guess this is probably a software bug/nuisance, rather than a hardware issue given that I otherwise don't have signal problems when I'm above ground. Today's my last day for a full device return, though, so I'm wondering if anyone else can chime in.