Drive Dropped Out of a 3-drive Raid 5 Array

We have 3 WD Gold 6TB in a RAID5 in a HIKVISION NVR

Disk1 Functional WDC WD6002FRYZ-01WD5B0
Disk2 raid5 Array WDC WD6002FRYZ-01WD5B1
Disk3 raid5 Array WDC WD6002FRYZ-01WD5B1

Raid5 Status: Degraded Physical Disks: Disk2, Disk3

It seems to me that Disk 1 dropped out of the array.
Is it possible for a disk to drop out of the array due to errors and for the system to forget why upon reboot?
But most importantly, Is it safe to rebuild the array?
Any insight would be most appreciated.