This is a question between "you're totally safe" and "you should have, now there's a risk that..."

Shortly: I showed directly from my stick an Excel workbook that is about three times encrypted on a third party computer (typical computer class, almost surely no keylogger or such a thing there). I removed the stick and shut down the computer. The OS is/was Windows 7.

The question is: Even if in the "recent files" of Excel / Windows is listed this workbook, is there absolutely any way to recover the data now? Or am I 100% safe with this?

With "directly" I mean without copying it to anywhere on the hard drive (does Excel do some temp file to hard drive? RAM shouldn't be enough, right?) and without saving it while / after showing the file.

The three used encryptions are
1) Excel user authentication from AD
2) Excel own password
3) SQL Server 2008 user authentication

Quite safe

That's what I think too, but what would it take to be able to parse it back together? I really would prefer some "use of equipment that no norrmal person has".

And yes, I don't even need the exact way, since I hope there simply isn't one.

Assuming you open "secretStuff.xlsx" from the USB Stick, The Excel creates a temporary file called "~$secretStuff.xlsx" in the same folder on the USB. This is the temporary file that Excel uses to work on until you save it(or autosave saves it), and then it is written to the Original. The only other place it exsists is in RAM.

Also, you are actually misinterpreting the word "encryptions"

The three used encryptions are
1) Excel user authentication from AD
2) Excel own password
3) SQL Server 2008 user authentication

These are protections, if you had the file, IMHO only the excel password may offer some protection, but there are also ways around that such as crackers.

I believe the answer to your original question is as sathyan says and is quite safe.