Q: Have I been hacked?

No hacking. The Mac just though one monitor lost power/signal and moved everything to the other monitor.

Try:

- Try resetting the SMC and NVRAM/PRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

https://support.apple.com/en-us/HT201295

About NVRAM and PRAM

https://support.apple.com/en-us/HT204063

- Disconnecting/reconnection the problem monitor

- Another monitor cable

- Another monitor

Hi Michael, do others have physical access to you Mac? Sometime a little movement can trigger Screen Sleep to wake.

It's doubtful anything at all was done.

Possible this is an issue with the GPU or the detection of the monitor through its own power management, and it sounds like the monitor is switching from secondary to primary, not conclusive to hacking. If by some odd chance it was Hacking it would require you or someone with your password to install software that requires you administrative rights and physical access to the computer.

Along with all other suggestions you may consider resetting your monitors to the factory defaults, If your monitors have a reset option, and make sure your Mac is up-to-date with security and OS patches from Apple, which are actually very important if you are concerned with the security of your computer.