Ok, I must say I didn't know about this, it is not clear from the help file.
I also believe that unless you're the one updating the file from an external application at the same time, it would be impossible to know how a third party updated your file. However maybe this is good for communication between two mIRCs?
It seems to me that most scripts using file handling currently don't want external application to update their content (probably not even read it) and would want to use the recent -x switch of /fopen.
In any case, it looks like the optimization of /fseek I mentioned is possible to do when -x is used with /fopen, what do you think?