Are there steps I can take to rule things out eventually leading to a root cause?
I am currently using the latest Dirty Unicorn ROM on a Nexus 5.
For the same of argument, consider the case where the phone is down to 50% and 'Android OS' was responsible for 25-35% of that drain and the 'Screen' was responsible for the bulk of the remainder.
Typically I would expect to see ~25% Screen drain after a good bit of usage but AOS would not also account for that + some. In my case, I am seeing twice as much drain as I would typically see due to this.
I am suspecting it is due to an always connected VPN but I would like to be able to get definitive evidence. The VPN app process is using a negligible amount of CPU.