SOLVED!

So, just a recap: when going to sleep, I let the device idling with the same usage scenario as always, and 8 hours later, Battery Usage was showing 12mA, and a 10% decrease in battery life. It used to be 5mA before, with a 4% decrease. That issue happened for almost 3 months, so I'm sure it was something 'big'.

So, what I did? I simply let the phone on, draining battery until it shut down itself. Then i recharged it, and everything was back to normal. Tested it this night and got a 4mA avg idle usage, and a 4% drain in 8 hours.

Maybe that's a new way to try fixing high battery usage on idle? Pretty strange, IMHO. I actually have no technical idea why this solved my issue. Any thoughts?

Thanks everyone who tried to help in this thread!