As I state in the subject, this topic is about OS X / macOS only. It's not my intention to bash Prey in any case -- I just want to clarify if I have it right how Prey works regarding this OS.
SCENARIO A - Unencrypted boot partition or who in their right mind would do something like that!?
In the case the partition is indeed unencrypted, Prey will normally load at boot time being ready to hail home as soon as the system can reach the internet. For this to happen a user account is obviously needed; ideally it would be a limited account, without password, something like "Guest account" or "Test account". This will make it easy for a potential thief to use the computer long enough increasing the chances of localizing it.
SCENARIO B: Encrypted boot partition
Let's say you are a conscious user who understands that no matter how expensive is the fancy piece of hardware you love to use, the information contained there is even more valuable than the computer itself. Your work stuff, family stuff, access to all your online accounts -- everything. Like, you know, most if not all of us usually happen to have in our main systems. Having the disk fully encrypted is a must.
The problem with this approach is that in order to let the system boot - and load Prey - you firstly need to enter your encryption password. Hence, Prey will never load and will be rendered useless. If you encrypt your storage, forget about using Prey.
GUEST ACCOUNT: NO USE
The way the guest account is implemented in macOS makes it impossible for any normal application to interact with it. This is good and bad. It is good because there isn't an easy way to tamper with the account. It is bad because there's no official way to install useful stuff like Prey. So, again, if you enable the guest account as a kind of a bait-account so the potential thief can use the computer to give you enough time to trace him/she back, it won't work because Prey won't be loaded.
TL;DR: Prey is pretty useless unless the partition from what the system boots is unencrypted and a dummy, limited, bait-account is created.
If the thief is someone with at least two fingers of forehead, first thing he/she will do is to boot into rescue mode, format the internal storage and iron a new copy of the OS onto it.
I'm not sure if at this point Apple could be of any help but it would be nice if they can provide the IPs associated to the Mac S/N. I mean, they have to know them, at least the IP address used during the OS installation. Pray (with an a) that the sucker isn't behind a VPN proxy for that.
So, that's pretty much it. Am I missing something?