I'd venture a guess that it's stored on the sd card, when you put a new card in you had to re-enter it.
Could have lost it before due to power loss creating read/write error on the card? Just a stab in the dark..
I think you may be correct as I think it only happened with new cards. Once the key was entered, I swap that cards when I make major changes to my favorites or when I update the master database and the key sticks. I do that sometimes several times per week and have not lost a key since.
I just ordered a 2nd 436HP so I guess I'll experiment with that radio when it gets here and before I set it all up.
edit: At least the ProVoice key is actually easy to enter. It appears to only allow selection of the HEX characters from 0 through F so going through the key selection choices only displays the HEX values and skips the alpha keys above F as well as the non numeric characters for ProVoice key entry. That was a smart move on Uniden's part.
One thing I noticed on the MyUniden My scanners screen is I'll get another key generated if I go in and change the place or date of purchase in the fields one can still edit even after you have purchased the extreme upgrade key for the HP1.
Not sure if that is still possible nor if the key changes on the x36HP line if you edit any of those fields today.
When I first noticed it with my HP-1, I thought I was going crazy as it was not the key I had on file from my original extreme upgrade purchase.
Somehow my HP-1 had lost its extreme upgrade option after I'd got into one of the factory test modes. Whatever test mode I got into, wiped out my extreme upgrade but I learned later that the original key as well as the new keys all worked to restore my extreme upgrade thankfully!
I hope that is still the case with these x36HP models if one looses their key somehow and the MyUniden site displays a different key.
Uniden must generate a key based on the radios ESN and Checksum as well as the date and place of purchase but all keys worked on my HP1 after I discovered this. I purposely played around with changing the date of purchase as well as the location and came up with at least 7 different keys before I gave up. They all worked.
As long as there is no test mode that changes the ESN or Sum values, any key displayed on their site should restore the paid mode settings in the radio.
I do wonder if anyone has found any undocumented test modes for the x36HP models but if so, I think I'll stay away from them as the loss of my extreme key for my HP1 did scare me until I found that any key generated restored the paid upgrade!
It was really pure accident that I found changing the location of purchase also changed the extreme upgrade key. Somehow when I initially registered the HP1, I'd used the store name for the city of purchase. When I noticed that and corrected it, is when I noticed my extreme upgrade key had also changed from what I'd filed away.