What devices should do, and what they actually do, are two very different things! As you've probably heard me mention countless times, many of those screens get information in a proprietary manner directly from the modem; apps like SignalCheck can only get what the OS is reporting.
I did find your earlier reports, and as I guessed, the OS was reporting valid signal strength. I will try to figure out a solution.
Service mode shows the SIM as having no service, so I would think the device should report that. I believe I sent a diagnostic report a little while back—can you check to see if you have it?
Interesting, that doesn't happen on my P7. Can you send me a diagnostic report the next time you see that please? Hopefully the phone isn't actually reporting the -115, that will be tricky.