[CentOS] Very strange problem i have faced in my 2 years carrier

>> I have seen vi do this action when it didn't understand a keycode on teh
>> terminal you are using properly... change the case of a few letters next
>> to the cursor.  But IIRC that was busybox vi.
>>
>> Is it crazy to propose someone opened /etc/passwd in vi, and saved it
>> out without noticing this had happened?
>>

If you suspect your box has been rooted, then perhaps it is time to do 
some checking.

rpm -Va

Also, have you ever updated the box?