Is a portable apps and a thumb drive secure

Yes, because : - portable apps don't leave anything behind on the host
if they are terminated correctly
No,in your situation, because : You can not run a program on a computer
without it being detectable .. If your employer has tight
security there is a good chance they are monitoring
what software is running on their network.
They probably also have rules forbidding the use of non-authorized software .
If you do it anyway and get caught it could easily cost you your job ..

There's secure and then there's secure. If you mean secure from a deep forensic analysis of all hard drives finding any trace that you ever used the machine, then no. If you mean secure in that obvious personal data is not left behind, then yes.

You probably mean something in between. Only you can know just how many traces you can tolerate. We can help you figure out where the risk is, and how to mitigate some of the risks. For example, if your program uses temporary files in the user TEMP directory, then deletes them when it is finished, that may or may not be enough for you. If you need to be sure that the data can't be "undeleted", then you may want to either redirect the TEMP directory to your removable drive, or run a "free space wipe" program to make sure that the data is gone. If you just want to make sure a simple undelete won't find it, overwriting it once with random data might be good enough, whereas if you are afraid the NSA might try to analyze the magnetic fields to figure out what data was there before it was wiped, you would want to use a more strict wipe, overwriting with random data several times to make sure the magnetic traces are ambiguous.

You've asked this question in several places around the forums. There are other answers at those other places. Like https://portableapps.com/node/10526#comment-64253