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Just pulled a very old MiFi device out from the back of my bedside drawer where I know it's definitely been sitting unused for at least 4 years.
While I was opening it to see if there was a SIM card inside I accidentally hit the power button - it immediately burst into life and showed a half charged battery.
WHAT?!!
After 4 years how the hell can that be?!!!
It was switched off rather than in standby?
If it's a lithium battery of some sort and at close to it's storage voltage the it'll sit ok like that for a long time. I have an old Nokia phone (a late one, not one of the indestructible models of legend) that I only used for a few days while my old phone was being repaired. 6 years later it was rediscovered and switched on like it was only yesterday with two out of three bar of battery remaining! Took nearly a week to beep asking for another charge and is still going strong now. Used as an emergency phone these days, charged once in a blue moon.
Whilst getting our house rewired, we bought a new doorbell system because we assumed that the doorbell was hard-wired. When all the old electrics and fuse box was finally removed. The old doorbell was still working. Yep, battery powered. Checked the batteries, they still had a full charge.
Only being living in the house for 23 years......
We have a Drayton wireless thermostat / programmer for our central heating. It runs off (I think) 4 AA alkaline batteries and is on constantly (LCD display and sending commands to the receiver at the boiler).
It was installed getting on for 16 years ago and I've only changed the batteries once (and that would have been quite a few years ago now).
Bunch of lightweights : This Battery Has Lasted 175 Years and No One Knows How
https://www.vice.com/en/article/9akg9d/this-battery-has-lasted-175-years-and-no-one-knows-how
Rice cooker - It has a LCD clock on the front of it that stays on regardless of whether the thing is plugged in to actually cook rice. I've never changed a battery or actually investigated what powers it. So as far as I know could in fact, be magic. It's also surprisingly accurate over about 8-10? years or so that we've owned the thing.