If it were me, I would try this next:
1. search the entire shared drive for *.ppt; you don’t need access to the entire network; you only need access that mirrors that of the employee who created/saved this data.
you will probably get a bunch of useless results, but you will also rule out that the PPTs are no longer on the network.

2. explore the option of a data restore from archived backup tape; if possible, go as far back as when the person was still employed and working on this particular project. tapes this old might be stored offsite; check with your IT manager for those details. Lots of help here: https://forums.techguy.org/threads/p...ening.1129888/