MISSING files in 3.7.5!!!

In a particular session I created a "favourite" folder into which I dragged all tagged images.
This folder has, after two days, mysteriously disappeared (it shows in red letters), and ALL the files in that folder are gone!

- I have performed a search for the missing files by name with no result
- I have NOT deleted anything (even if I had, I have not emptied trash)
- I have done NOTHING in the meantime - the folder and files were there for two days after the shoot en were then gone.
- This has happened to me once before 8 months ago. Then a data recovery company could not find the files even after restoring 50 GBs of files I had deleted months before; we ran a thorough check of hidden files.
- This is now a different Mac running Leopard (that was running the previous OS). Then and now the computer appears to be in perfect working order, no other files and programs are affected.
- The "memory available" indicates that the Gigabytes lost are still somewhere on the computer, if that is any indicator.

Has this happened to anyone before, and is there ANYTHING I can do to recover the files? Stupidly I had not backed this session up yet, and now I'm stuck with all the files that I did NOT move to the "favourite" folder..