Explain the Time Machine backup when no drive is attached, Explain the Time Machine backup when no drive is attached

I'm traveling now and just noticed that my Time Machine is still turned ON. What caught my attention is that it is still functioning without my Backup external hard drive attached. It seems to be creating Time Machine backups anyway... and is storing them in an invisible simulated hard drive called "Backups.backupdb". What is this? How is Time Machine doing any backups without my normal designated backup hard drive attached?