The three day thing is that after being away from your phone for three days the tag will emit a sound each time it is moved.
So the tag would continue to give a location ping to you via every passing iPhone until the battery dies (c. 1 year) and only if they detected movement would they emit the sound to tell someone nearby they are lonely and missing their connected iPhone. This does mean if you dropped your keys in the woods while out walking your non-existent dog and didn’t realise. You could still locate your keys a month later provided an iPhone or two went within 200 or so meters of the tag. But if after three days someone with a dog disturbed the ground, your keys would ping and they could use NFC (inc. android devices) to read your missing device message. So yes, potential to stalk someone for three days, including a spouse you suspect of playing away.
This has good spousal tracking though as the spouse will likely have your address as home. On arrival at home, instead of sending the you’re being stalked alert, the tag will start talking to your phone again. It will probably never tell them they have been stalked. The moral of the story being, check your pockets and bag compartments carefully if you are playing away.