If you're using Filevault for full disk encryption you can't have Prey installed and use it on an open guest account. You can have a guest account and use the iCloud 'Find My Mac' though.

But that's exactly what I did on my last Mac (which wasn't using Filevault).

They won't need the guest account with this product.

You use the guest account as a honeypot so they open the Mac and use it for long enough (without wiping it) for you to get a wifi connection and can get the location with Prey :-)

Sure, that's a best-case scenario. How about no WiFi in the vicinity and a couple of accidental knocks on the phone?

Haha, yeah - in that case you're screwed ;-)

Edit: although ideally Prey will still be running. They'll just be able to run amok in the meantime...!