What the scanning dots under the blank cellular strength triangle means is that your phone is looking for but not finding cell coverage. And, if your phone doesn't find cell coverage, it will make calls using wifi. You can tell whether you're on wifi by looking at your phone just after you've made a call. If you see your wifi SSID at the top of the screen, then you're making a wifi call. No problem, Fi is fine with making wifi calls. So you don't need cell coverage to make a call (or get a call or get a text or send a text) on Project Fi.
So the answer is yes, I see the scanning dots at the bottom of the triangle when I'm in an area that has no coverage, or where there's no TMobile but there is Sprint. The triangle will scan for a while, looking for TMobile, and then pick up the Sprint signal. That's what it does at my house, where TMobile is uselessly weak, but Sprint is fine. Or I can use wifi calling by putting the phone in Airplane mode and turning wifi back on.
Fun stuff, I really like Project Fi.