Does the ST-E3-RT (or YN-E3-RT) Transmitter report when a slave Speedlite goes offline?

I'm fixin' to buy me a Yongnuo YN-E3-RT Speedlite Transmitter to control my two Yongnuo YN600EX-RT Speedlites via radio. I currently use my Canon T4i's built-in flash to control them via optical which is only a one-way communication. This has the advantage of being able to include my Canon 430EX II Speedlite in the mix, but since it can only be controlled wirelessly via optical, I'll lose the ability to control it with the YN-E3-RT. I don't mind; it's on loan anyway.

I just answered my own question. Oddly enough, it was just as I was about to fall asleep. I was thinking about my conversation today with a salesman at my local camera store. He couldn't answer this question either but said he had one ST-E3-RT Transmitter in stock and if I brought in my two 600 Speedlites, we could test out what happens when you pull the batteries out of one.

Well, I was reading up on the new Canon 430EX-RT III Speedlite trying to determine its wireless capabilities, which was harder than you'd think. It turns out that it can be a master or slave as wireless radio, but only a slave as wireless optical. And for $250 it's a great small powerful lightweight flash to have around especially on camera. If you don't care about master optical wireless, it's a bargain compared to the $300 price tag for the ST-E3-RT Transmitter. (Actually, I must mention a bug I found in the 430EX-RT III: its AF assist beam will only work for the camera's center AF point. That's a big flaw.)

Of course, I'm still planning on buying the Yongnuo YN-E3-RT Transmitter for $76. But, of all the research I've done, it didn't occur to me until I got sleepy that I have the answer right here in my apartment. I own two YN600EX-RT Speedlites!

The YN600EX-RT Speedlite can be either a slave or master for RT style radio wireless. Essentially, it can mimic the ST-E3-RT Transmitter. So, I put #1 Speedlite on my camera and set it to master radio wireless mode; the link light went blue meaning it's searching for a slave. Then I put #2 Speedlite in slave mode; both link lights went green meaning they're connected. I told the master to use manual power which then displays a row for each flash; the first row being group A, #1, and has no bolt because its not connected wirelessly, it is its own self. The second row is group B, #2, and has a bolt. I'm ready to test!

I pull the batteries from #2: nothing happens. #1 still has a green link light and row B still has a bolt; the master still thinks #2 is online and ready to go. Even after taking a picture it still thinks it's there. Not good news. So, I try something else. I put the batteries back in, #2 gets the green link, all is okay. It take a picture; it's fine. Then I turn off #2 and #1, the master, sees it's no longer online, the link light changes from green to blue, and the bolt on row B, #2, disappears. Can you friggin' believe it?

So, the bottom line is the behavior I want it to do when the batteries die happens when it's turned off but not when the batteries die, go figure. It must send a signal before shutting down to its master telling it that it's going offline. My only last hope is that if the batteries are actually dying, and not that I abruptly opened the battery door, that it would send a signal before it died completely that it's going offline. The only way to test this is to put low batteries in the slave and start shooting. And if I get the itch to test this theory, I'll let you'all know.

I had this crazy idea of doing a Magic Lantern on Yongnuo Speedlight equipment taking advantage of the USB port for upgrading the firmware. Reprogram both the YN-E3-RT & YN600EX-RT to send battery level info and when low toggle between the bolt and battery level on the row for that flash on the LCD display of the transmitter.

IT WORKED! I tested it 3 times. I put nearly dead batteries in #2 slave set to full power. After a few shots, the batteries die, the LCD screen changes to just a big blinking battery, it beeps 3 times, then shuts off. At that point, #1 master link goes from green (linked) to blue (unlinked). The bolt on the row of that slave never reappears because the bolt indicated ready. It would be nice if the master beeped, though.

Some additional info not found in any manual:
1. The master's link light stays green when it's connected to at least one slave. So, if you have two slaves and the batteries die on one, it stays green. The only way to tell on the transmitter (YN-E3-RT) that you lost a flash is the bolt icon never appears on the row of your flash. There is an exception: each row on the transmitter's display shows a group (A, B, C, D, or E) and if the flash that dies is in a group with another flash that's fine, you'll never know because the working flash will indicate it's ready and the bolt icon will appear for that group's row on the transmitter's LCD.
2. Canon's link lights are red when not connected where Yongnuo's are blue. They're both green when connected. So, if you're calling Canon support (800) 828-4040 for help with your Yongnuo products and you're representing them as Canon, don't tell support it's blue; they'll freak out and try to find out everything about your illegal equipment. He started asking for the S/N.
3. The Flash Ready/Pilot Test Button below the bolt icon on the master turns red only when all flashes have recycled and are ready. So, if you've got five slaves out there and recycle time is getting slower and slower and the Ready light is taking longer and longer to turn red, you know you've got dead batteries in a flash. You can look at the display to see which is the last to display the bolt icon: that's the flash that needs its batteries changed.