Thanks for the link.
I just tested with my work badge (Mirfare classic with nothing store on it, they just use the ID).
The door beep, like if something is found, but no error nor green light, just a bip.
It does the same thing with the app closed (I didn't test before starting the app so I don't know if it changed something in the NFC chip).
I don't really know what is possible with the NFC host mode on the nexus 5 but I think it will not be possible to spoof the tag ID :'(
I'll test later with another NFC phone to see if I can emulate a contact card from the nexus 5.