It‘s likely it would’ve been dead if it was left somewhere unused. Batteries die if you don’t recharge them, especially on an old used iPhone. And since it’s likely an old model we’re talking about? I don’t think anyone, if found, would have any incentive to replace the battery so they can turn it on, if this eases your mind. This is why an iPhone 5s and another 6s in my drawer are completely dead. I wanted to fiddle with them but repairing them makes no economical sense.
Regardless, even if that wasn’t the case, I wouldn’t worry about it. If it had no passcode the first thing they’d do is jump to reset it so it doesn’t ping your Find My (regardless of whether it was actually enabled or not) if they had any intention of fiddling with it or even keeping it.