Once possibility is that your N810 may be crashing and rebooting whilst you're not using it. It reboots into an unlocked state. I concluded that's what happens when I find mine unlocked in the morning. Mine seems to crash and reboot pretty regularly - sometimes multiple times a day and sometimes only a few times a week. No idea why - I haven't really investigated as it's rarely been a problem (except when it crashed when the GPS was telling me where to drive!). Just open Xterminal and type 'uptime' and it will tell you how long it's been running. If it's shorter than the last time you manually booted it, it has crashed and rebooted itself.