Depending on what you set as the timeout for the Opus will depend on what will happen. I'll stick with the default setting, which is 10 seconds.
The PC is on, and you switch off your ignition. If the car is switched back on in less than 10 seconds, then the shutdown timer will be cancelled, it won't do anything. If it's over 10 seconds, then it will send the shutdown command to the PC. Depending on what you've set the PC to (i've got mine set to Standby, as it uses so little power) will determine how long it takes to shut down. Mine is about 3 seconds at most after this 10 second timeout is reached.
Then, when the ignition is turned back on, after 5 seconds, it will start up again (with mine, it it playing music within 2 seconds of this event, yours will differ depending on how you shut it down). You switch off the igniton again, and it will shut down again.
Say you've set it to hibernate, which takes 10 seconds. If you switch off the car, it leaves 10 seconds, then it starts to shut down, but then 5 seconds later the attendant switches on the igition, it will shut down, then start up again as soon as it's gone off. If it then takes another 15 seconds to resume from hibernate, and he turns off the ignition before it's loaded, then the Opus will send another pulse to the motherboard. However, this may or may not set it back to shut down if it hasn't fully finished resuming Windows.
Maybe your best bet is to put a SPST switch inline with the remote line to the Opus, so that in one setting, it is switched on your ignition, or you can switch it off so that it never comes on with the ignition. When you get out the car to give it to the attendant, flip the switch to "Off", and then it won't ever start up while the attendant is moving the car around, then when you get back in, flip it back to "On", and it will work as normal.
The other option it to set the timeout to 10 minutes, then it will wait for the igntion to be off for 10 minutes before it will initiate the shutdown. This shouldn't drain the battery too much at all