most common problem seam to be double coils. if this is not your problem, open datatable for those bits and click on usage. used bits will have "X" instead of "." but this only shows if bits are used in program (contacts, coils). if you look on the left side and see "F" "W" or "FW" then you know that the whole word (including every single bit in it) could be changed somewhere else in a program by instructions like MOV, COP, FLL, CLR etc.
even if you don't find anything like that, you have to check if this plc is connected to other devices (HMI, PLC or PC for example). This connection is not always obvious if the PLC is on a network. ANY node on a network can write to plc and set or clear bits. if that's not what you want, you can go offline and change setting for that particular table (click on properties and look at the bottom of the form, you will see something called protection).....
