In high end cars, to fully protect the alarm-system brain, we would fully remove the dash, and bury the system DEEP. Sometimes we put a "dummy" unit and wiring in the kick panel to send would-be thieves on a wild goose chase, wasting their time. We'd also install a dummy valet switch, as that's one of the best ways to track down where the brain is. We used units that used codes for valet switching, not a physical switch.
We would not put the unit under the hood, as it's exposed to too many environmental conditions.
Why do you want to locate the unit? The more people that know where it is, the greater the compromise to your security.
1998 C230 "Black Betty"