I prefer self bondage these days, (generally speaking) where one isn't anchored to something that can't move, like a tree or series of trees. I have in the past hidden an emergency key in a group of sticker bushes some distance away from where I am playing, being forced to hobble or hop to my key and get scratched up to gain my freedom if my planned escape fails. I have also hidden a key to the locks about my legs relatively close by, but the key to my cuffs quite some distance away providing a further adventure to gain my freedom if my plan "A" escape fails, (keys that don't drop where they should are considered to be lost for good in my mind). When on such adventures every sound in the forest is magnified to my ears as my cuffed behind my back and naked form is about as vulnerable as it can be.
Once I had a black bear interrupt my fun, I forced to make my way away from my play area to hide in the woods until I thought it was safe to return, all in all an exciting adventure, although not in the way I had anticipated. Another time I heard voices that sounded much closer than they actually were, I getting myself misdirected in the woods naked and cuffed in my attempt to hide, only to have it start to rain and I soaked to the bone before I got back to my car and clothes.
Anchored in place is incredibly exciting to me, the Perils Of Pauline factor and the resulting fantasies extremely high, as are the risks. I have used a padlock frozen closed with the key in the lock as a release mechanism, with my back up being an ordinary padlock with the key intertwined in the chain someplace where I can reach it. That is my emergency release, and if I have to use that one I have a price in my mind for using it. I once used it, not because I was stuck, but because I got bored and wanted out, my self inflicted punishment to spend the entire next adventure, (all day long) naked and cuffed in the woods on the errand of hiding a set of keys for a future play date.
One could also have a jar with various emergency key punishments listed, you having to chose one from the jar for using the emergency escape. It could be anything from discretely going commando at work the following day, or something hugely embarrassing like going to an adult shop and buying the largest sex toy they have.
Involving a trusted friend with a time to worry by and location is by far safer (as a very last resort), but has it's own potential consequences, and adventures, and doesn't depend on a technology fail.
Just my thoughts, please play safe, Jackie.
P.S. I tend to trust mechanical things over electronic ones, ironic, or possibly not if you knew what I did for a living.