What Windows notes about USBEvery time a memory stick or portable disk is inserted into a Windows machine, Windows logs it, along with the serial number and how long it remained connected. A memory stick appearing for the very first time at 2am on a Sunday, a few days before a resignation, needs an explanation.
Webmail and cloud storageWhether OneDrive, Dropbox, Google Drive or webmail in a browser was used, each leaves residual traces on the machine: sync database entries and cached tokens, browsing history, and uploads that were interrupted part-way. Logging off and tidying up afterwards removes very little of it.
Wipers and wholesale deletionDestroying evidence tends to generate even more of it. What remains is the name of the software used, the time it ran, and (typically the deciding factor) everything it could not reach.
The order is the evidenceSingly, each item likely has an innocent explanation. Lay the events out against the clock (files collected, memory stick in, upload completed, wiper run) and coincidence stops being the obvious explanation.