Using jenkins 1.420, jenkins offline node has its workspace folders modified when the node is marked as offline.
1. mark node as offline
2. wait for tasks to complete on node.
3. create a folder that doesn't have a matching job description string in the workspace root.
4. put some files or do some forensics in that folder.
5. wait.
6. confirm java slave agent is still running.
7. Note that eventually folder will be silently pruned, losing data.

It was a surprise to me that jenkins deleted my working folder in the workspace (that didn't match any existing job) when the node was offline.

Expect that when node is offline, jenkins master does not modify the state of the jenkins workspace directory, and that offline nodes queue their garbage collection or workspace cleaning for another time (when node comes back online, or next reasonable interval).