In theory, you could create a simple AppleScript script that is executed every time a specific folder is opened, and then that could just make an entry in a log file somewhere for you to check later. However, I'm guessing anyone who has to ask this question isn't going to be up to the challenge of creating such a script, and you'd also have to figure out a way to eliminate possible false-positives from things like Time Machine and Spotlight.
So, long story short, if you don't want someone accessing certain files, keep them on some kind of removable storage that you take with you every time you get up from the computer.
I'm using a G5 PPC - OS-X 10.5.8

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