Basically it's impossible for a computer to change it's security settings (ie whether or not it needs a password to log in after waking up) itself. Therefore i'd look elsewhere to find out who has done this
Would say you are right to be anxious. At first guess it sounds as though somebody else knew the admin password, and has now changed it and also changed the security requirements for log in. Is this a possible scenario? If so I would be concerned about software that could have been installed without your knowledge (eg. spyware / keyloggers) as somebody 'could' be watching you. I don't want to panic you but it sounds very suspicious
Normally when my imac wakes up I need to log in - I usually log in on the admin account, however this morning it just woke up, not asking me to log in and all day it's not asked me to log in with my password
I just tried to change the admin password but it's telling me the old password is incorrect - it definitely isn't!
any ideas what's going on? am now feeling a little anxious ...