Will I Drain My Battery Leaving my Trunk Open?

You should be fine if you turn off the trunk and interior lights. If you want, you can also tape over the trigger to trick the car into thinking the hatch is closed. I don't have a Gen 4 so I don't know where the trigger is. It may be in the latch or probably a little rubber ribbed button. You'll know if you have it if you push it down and the interior light turns off.

 

There could be (unswitchable) footwell lights on as well. I know that's the case with our 2010 hatchback. And yeah, it's never a good idea to leave a door or the hatch ajar for a protracted time. The hood (over the engine bay) is the one exception to this rule.

I would spoof the hatched closed condition, basically fool the car. Look at the latch on the hatch lid, the bar that goes into the locking latch, and try to find something that could mimic it, say a carabiner:

 

I was working on my car and forgot the hatch open for a few days. That did drain the battery and I had to charge it overnight to get the car started again.

The battery might survive for a few hours but spoofing the hatch might be a better idea.

 

My trunk wasn't closed all the way since yesterday morning to this morning. Was maybe 20 hours or so. I didn't notice whether the trunk lights were on but the dash indicator showing the open trunk was on. I didn't notice any issues.

 

I know my last 3 cars (KIA, FORD and FORD) would turn them off after a while, and I thought I read something similar in the Prius Manual - 20 mins rings a bell. How about opening it and checking it after 25 mins to see if the light is off then?

 

FOUND IT

Manual - "If the interior lights remain on when the power switch is turned off, the light will go off automatically after 20 mins."

 

This is a third gen, so maybe different, and different locale too fwiw: but I wouldn't trust the car to shut down lights. I left one door ajar for 18 hours, and it severely depleted the 12 volt. The degree of protection afforded the 12 volt seems archaic these days.

 

The 12V battery in the Gen3 is quite smaller than the usual 12V battery found in cars of yestercentury. This is becasue the 12V battery in the Gen3 is meant to be used to turn on the computers to turn on the car and access the high voltage battery, than then feeds the 12V. This is why they say in the manual to never leave the car in accessory mode for a long period of time, but rather leave the car in D (and not in N!) to allow the car to use the engine to charge the high voltage battery, that can charge also the 12V battery.
In Gen4 I have noticed that the battery is slightly bigger, but didn't care to check if it is a "normal" battery again - and if so, why.

 

Yeah it'd be interesting to get some more spec's on the fourth gen's 12 volt battery. Is it Absorbed Glass Mat? Vented (likely not)? Regular post size? Dimensions? Cold Cranking Amps?