Are they any water moisture detection stickers inside the phone

what if condensation or water vapour triggered these sensors that are exposed externally, but its no water has actually entered inside the device, then these external stickers are a slightly misleading

They are misleading, but I think that's the point. The only reason they're there is to give the techs a way to weasel out of repairing your device while it's still under warranty (at least in my experience). Say you have a device that's acting up and needs repair. You take it in and the first thing they check are the moisture sensors. Even if it's still under warranty, if those sensors are activated they will claim it's water damage which isn't a manufacturer default and will refuse to fix it.

Even if your device has never been anywhere near water. Nice little trick huh. I've even heard that just having your device (say a phone) in the pocket of your pants on your bathroom floor while you take a shower is enough to activate the sensors due to all the moisture in the air.

Anyway, just my two cents.

Whao whoa whoa. Hedphone jack? That's balls deep in the motherboard of every phone I've disassembled so far. Is it near the jack, or physically inside it? If the latter, without a magnifying glass how would you even see it? In either case, that's still "inside" the phone for all intents and purposes.