He posted this after the fact and probably not from the location of the low signal.
Also, I would think low signal only registers when the battery usage for each unique app is done while the majority of the percentage used is during that one dot/bar period and not for just say 1% out of 10% total usage?
You probably have background app refresh running? With that bad of a signal at work, it'll kill ya.
I used to go to work in a really bad reception area on my 5s and after 9 hours I would be at 65% after a full charge and very, very light usage.
With my 6 and typically the same usage I was at 94% yesterday. Here at least, maybe the marketed new LTE bands is helping, and not just the larger battery?
I also have a 6+ and that phone in the same conditions leaves work at 96%.