Depends on what the po attached the alarm ground to. If the metal recp. Box is connected to a gounding system youre fine. ( modern residential systems have an established ground all the way back to the panel) and at that point the neutral and ground are connected together + this is supposed to be the ONLY point where they are connected to each other. The trouble is that older homes are usually so screwed up, it trakes an electrician to figure out if things are "right"
You NEVER permit two distinct, separate paths to ground. Verboten! That's a recipe for noise, to create an antenna for AM radio signals, feedback, for all sorts of problems. Grounding is always done in a star or tree configuration with one and ONLY one path to ground from any given point. Choose one wire and sever the other. Signed, an EE.
wade is right, only one gounding is the rule. and the ground socket on a recpt. is fine ... and none of the electrical system is suitable for lighting protection( for the structure). .....first reading of the post had me thinking that the po had simply rerouted the ground to the metal box. sorry for any confusion my misreading caused
In the past I have seen some sensitive electronic equipment that required a dedicated ground rod (and NOT connected to the main ground). This was in a manufacturing plant, and the main plant ground system was messed up (it had voltage on it and noise). Technically against code, but it wouldn't work otherwise. In a house with a proper ground, probably not an issue. Plus electronics today are more tolerant of noise.
A separate ground rod better be very separate. If any equipment can access both ground rods and there is a nearby lightning strike, the potential difference between the two ground rods can be very damaging. It's still against code. It says all ground rods have to be bonded with some very heavy wiring.
In that case I believe you have to isolate the equipment from the AC lines as well using a 1:1 transformer. Done properly that should meet code.