So there is a peculiar behavior that players can use to determine when an enemy transitions to T2 and what T2 they're going for (or if they're upgrading their mexes, reclaiming T1 pgens etc).
1. Here is a screenshot I took from a match between Swkoll and KEYSCORE. What you can see here is how KEYSCORE's base looks like to the observer:
2. This screenshot shows what Swkoll can see shortly after he had scouted it:
Now here is the interesting tidbit:
3. About 10 minutes later this is what KEYSCORE's base now looks like (Swkoll hadn't been able to scout for quite a while at this point):
4. But here is what Swkoll can see on his screen:
Now via process of elimination he could easily make the correct assumption that KEYSCORE had upgraded his other 3 mexes and more importantly made the upgrade on his air factory. To me this would be very useful information so I could start throwing down some Flak and maybe put a few more Anti-Air units into my Army.