Yes, and googling they exist, but then you have to have a special expensive bag to carry your phone in rather than your pocket.
That’d still leave the possibility of the phone recording and storing information to be uploaded as soon as it’s out of its cage.
Here’s a $4 solution.
The same would be true if you plugged it in, though.
It seems unnecessary to listen for the adapter when simply getting power can easily activate something. Not in full, just enough to allow the circuit that negotiates charging which could then keep it going.
If the device does simply keep using up the charge in the battery forever, then it would deplete to less than needed to run at some point. And then charging would be impossible. It just logically makes more sense that the charger’s power can activate something.
And that fits with some Laptop stuff I’ve watched. Plugging in the adapter does very quickly get power in at a very low rate, and then stuff switches on that negotiates the power.
When the battery is dead (too low to do anything), plugging in a charger will trickle charge the battery up to a minimum level using a hardware circuit.
Once it gets to that level, it can display a sign of life (SoL) on the screen and listen for the power button. It can also run part of the charging system so that it can negotiate with the adapter and charge at a higher rate.
I have a stack of 5 old iPhones (going back to my first iPhone 3). I charge them every month and turn them off (power down). The most any battery goes down while off for a month is about to 88%. Usually, higher.
They might be sending out occasional bluetooth alerts while “off”. But I would think constantly listening (and hence pinging) the cell tower(s) would eat a lot more battery than that, if a typical phone is lucky to go 2 days on a charge. So likely, “they” cannot activate your phone on demand.
OTOH, if “they” have implanted malware, or activated the part of the phone that Steve Jobs put in there that’s wrapped in tinfoil, then all bets are off - but likely, you’d see a significant drop in battery life while powered off.
My understanding is that standby power when off is in the microampere range. To put that in perspective, if your phone has a 3000mAh battery, and uses 50µA when off it would take over 6 years to discharge the battery. By which time the phone is obsolete. Plus batteries self-discharge slowly when left alone anyway.
I have witnessed an instance of (what apparently fits the facts) “they” attempting to turn on the phone microphone to listen in, but accidentally activating the speaker as well! The loud static was hardly stealthy.
You should not charge them to a 100%. Charge them to about 60%, recharge when they have dropped to about 40%. The battery will last longer that way.
If Steve put his super secret spy device into iPhones to spy on you while it was “off” he’d hide some of the battery capacity from you so you wouldn’t notice the power draw.
Years ago, Mark Zuckerberg gave an interview and people noticed that he had put tape over the camera on his laptop.
So, yeah, I think people can hack into Internet connected devices and watch you,
Zuck is a higher priority target than you are.
If I was the CIA, or Google, I’d totally hack Zuck’s laptop. Yours? Or mine? Ha ha, it is to laugh.
Computers, sure you might see some things, maybe. But a cell phone? They live in pockets.
Good point.
That’s my basic take on this. I just don’t think I’m important enough anyone would want to spy on me.
But..but…but they might get kick outta me tryna get my Siamese cats to put their faces by my face so we can have a selfie.
Been trying since I got them. Ain’t worked yet, but I have zillion tries. Some are pretty dang funny. I must say. 
Some were dangerous, it turns out.
They’d be wasting their time watching me. Nothing to see here.
Yeah, I recall a scandal some years ago where it turned out that a school was using school-provided laptops to spy on students at home. Including in their bedrooms.
Or in other words:
Nope. You don’t have to be important at all for people to want to spy on you. There’s plenty of people who’ll do it just because they can.
Don’t know many teenagers, do you?
From what I recall - the laptops had remote control software installed for managing and trouble-shooting, like many enterprises. But - it could show what the camera saw for the tech support person who connected remotely (with no warning or indication for the laptop user) The incident was some smart-alec kid who apparently was using Mike’s candy in front of his laptop - at school - pretending they were drugs; likely aware of the camera issue, because apparently he got suspended for drug use. The father raised holy hell about the issue, and the question was what safeguards were there about the school tech people (or anyone with the monitoring software) being able to view the camera anywhere, anytime, including when the child would be at home, in a state of undress, as long as the camera was live? Obviously, nothing prevented that, no restrictions and no warning from the software, and that was the problem.
It was something called TheftTrack, designed for covert surveillance and tracking. Outright spyware that let them both watch the students and browse what was on the laptop. Notably they lied repeatedly about it.
[T]here is absolutely no way that the District Tech people are going to monitor students at home. … If we were going to monitor student use at home, we would have stated so. Think about it—why would we do that? There is no purpose. We are not a police state. … There is no way that I would approve or advocate for the monitoring of students at home. I suggest you take a breath and relax.
It’s a long chunk of the article, but instructive as to how far they were willing to go and how intrusive they were willing to get. And how that spying on students was the intention from the start.
DiMedio said “the district did not widely publicize the feature ‘for obvious reasons’”.[22] She reportedly declined to tell students about TheftTrack because doing so could “defeat its purpose”.[22]
Perbix said that when “you’re controlling someone’s machine, you don’t want them to know what you’re doing”.[32][33][25][41] He praised TheftTrack in a YouTube video he produced, saying: “It’s … just a fantastic feature … especially when you’re in a school environment”.[42] Perbix maintained a personal blog in which he discussed computer oversight techniques, including how to cloak remote monitoring so it is invisible to the user.[43]
And given that this was just a school, imagine how far a corporation or government is willing to go. I don’t think it’s unrealistic at all to worry about them spying on ordinary people through their phones.