I apologize for not getting back to you sooner. When I saw some of your responses below, I guess I just lost interest.
So, have you done anything to fix this in the last six weeks?
OK. Have you made any changes to your script to only check this file once per hour?
That didn't answer either question.

Is /var/media/ftp/look.sh dependent on anything on the flashdrive? I.e., can we copy it off of the flash drive into /tmp and run it from there (which would change the number of accesses to the flashdrive from once every four seconds to just once)? And, why is it on your flashdrive at all? Why isn't it in your $HOME/bin directory instead?
This is what made me lose interest in this thread. If it is how it is and you aren't interested in changing it, why should we bother making suggestions?
OK. Thanks. That helps.
No, that is not what I meant. Can look.sh return two or more lines containing a name like hans? (Presumably, this would indicate that two or more wifi stations have detected that hans is within range.) If a given name can be reported more than once, does your script need to report that a user appears or disappears, or does it need to report that the number of times a user appears has changed?
OK. I assume this is because whoever is editing alertlist.txt is careless with the shift key when adding names to the list. It sounds like this might work perfectly for you... if you do NOT want to reduce the number of times your flashdrive is accessed.