when does OS/400 or IBM i reset the "last used" date of an object?

All:

One of my customers ran into this, and I am at a loss to explain it (so far).

On their production LPAR, they ran a DSPOBJD of all objects in one production library to an *OUTFILE some time on Friday.

Now, looking at the data in that outfile today, they notice that many of those objects had the "last used" date containing blanks -- e.g. not used since it was last "reset".

Yet today, they also noticed that if they now issue DSPOBJD against any of those same objects that had a blank "last used" date, on the same production LPAR, and specify DETAIL(*FULL), they tell me they now see a "last used" date on many (but not all?) of those objects -- and the last used date shown is "20140124" (last Friday)...

What can cause this?

I know, for example, that restoring objects from a previous save, (from tape or save file), resets the "last used" date to blanks.

But, what other commands or actions will set the "last used" date?

I know that CALLing a *PGM will set its "last used" date to the current date, as will OPENing a *FILE ...

Are there any other commands that could have "touched" many objects in this one library, causing many of them to have their "last used" date set?

Thanks in advance for any ideas ...

Mark S. Waterbury