I have an RFID antenna that has been delivered with a flat metallic plate glued to its back. I don't know much about this antenna it arrived without any documentation, it is square and can read tags in every orientation (not polarised). I am wondering whether I could remove it or if this might have been glued for a reason (insulation? amplification?). I guess in a more general sense my question is: does a metallic object behind an RFID antenna change the readings?
I assume that OP means "change antenna parameters", not digital content of ID. Note that RFID "Near Field Communication" antenna in smartphones is frequently glued to phone's battery, Samsung phones (S3, S4, J3, etc.) have it. The battery case is essentially metal, and the antenna works just fine. So the metallic plate might limit the distance of sensing (my speculations), so two persons with tags approaching the RFID base won't accidentally interfere.
There were quite a few studies on this subject, "because metallic surface changes its radiation pattern, input impedance, radiation efficiency and resonant frequency", see this article, and sophisticated modeling methods were used to evaluate the plate effects.
To reduce the influence of metaillc mounting surfaces on performance of RFID installations, special flexible ferrite tiles are offered,
I would hazard to guess that the plate is inseparable part of the antenna design, and I wouldn't remove it.
Most known RFID methods are digital, and incorporate some means of self-consistency checking.
Therefore, the actual identity "readings" would not be altered. What would be altered (likely substantially reduced) would be the chance that any given reading attempt is successful.
If you have a reader that reports some kind of signal quality metric, that could be altered.
Ultimately your question is a bit too broad, as going beyond the simplistic question, the meaningful question requires speculation about the nature of the unspecified device.