We have a remote site deployed using raspberry pi 2's where 3 out of the 5 have ended up damaged / unresponsive. It is hard to get an exact idea from the client but we suspect that the board may have blown.
We know for a fact that they put the pi's on a PoE network and I would like to know if it is possible that this could have damaged some of the raspberry pi's?
According to the standard, the power source shall detect if a device is PoE capable and only then apply the voltage, so no raspberry pis can not be damaged IF everything works according to spec.
I've heard about power sources from major manufacturers not working according to the spec, so while unlikely, it's still possible, especially with older, more unusual equipment.
On initial power up Power Sourcing Equipment is designed to detect, and only supply power to the network device if it is identified as a PoE-enabled device. When first connected the PSE initiates a detection process that involves low probing voltages to sense the type of PD connected while avoiding damage to non-PoE network devices.
Unless all Pi devices are dead at the same rate when plugged in to the PoE network, it's probably not the PoE. If PoE were the issue, they would simply all die right away. Also, network damage would probably only kill the ethernet port on a Pi, or maybe the complete smsc usb+ethernet chip, but not the SoC itself.