2008 won't start after sitting for 2 weeks. CM, ECU, or Hybrid battery?

The "once or twice" matters, actually. Once is called ACC, twice is called IG ON.

ACC powers the infotainment stuff and the power sockets. IG ON powers all the things (but doesn't start the engine or bring the hybrid system online). To get the hybrid system online and drive the car, you go READY (push the button with the brake pressed).

 

The fuse is in that orange plug. Check for continuity between the two big terminals on the orange safety plug, it you have continuity, the fuse is ok, time to expose the battery and start checking voltages.

The delay between the ign sequence being engaged and the ready light coming on is the system waiting for precharge resistor to bring the voltage up to the same level either side of the contactors, then they will engage ..... the clunk clunk sound heard in the left corner behind the rear seat.

I think the next step is to expose the contactors and associated gear (BMS or battery computer as Toyota call it, the precharge resistor and some associate wiring) check the voltage between the battery side of the negative contactor and the battery side of the positive contactor, that will tell you if the modules in the battery pack are at least charged enough to bring the precharge circuit to attempt to charge the capacitors in the motor inverter, if it can't do that, nothing much will work except for all the 12v appliances .....

Here is the Google AI overview of how to expose the hybrid battery

AI Overview



+2
To access the Prius Gen 2 battery contactors, you must first open the trunk, then remove the trunk liner and cargo tray to reveal the high-voltage battery. You'll need to remove the safety plug and then carefully remove the battery cover by taking out the 10mm bolts or clips to expose the high-voltage battery and its junction block assembly, where the contactors (including the pre-charge contactor and the negative and positive contactors) are located.
Step-by-Step Guide

1. Open the Trunk: Open the trunk of your Toyota Prius.
2. Remove Interior Panels: Remove the trunk liner, cargo tray, and any other panels blocking access to the hybrid battery.
3. Locate the High-Voltage Battery: The high-voltage battery will be visible in the rear of the vehicle.
4. Remove the Safety Plug: This is a crucial safety step. Locate and remove the orange-colored service plug (or safety plug) by turning its lever and pulling it out, which will kill the high voltage.
5. Remove the Battery Cover: Take off the battery cover by removing the 10mm bolts or clips that secure it.
6. Access the Junction Block: Once the cover is off, you will see the junction block, which contains the contactors. You will see the positive contactor, the negative contactor, and the pre-charge contactor (located under an orange bracket).

Important Safety Information

• Always Disconnect the 12-Volt Battery First:
  Before you even think about the high-voltage battery, you must disconnect the negative cable of the 12-volt battery to prevent accidental power surges.
• Use Insulated Tools:
  Always use insulated or coated tools when working near high-voltage components to minimize the risk of shock.
• Be Aware of High Voltage:
  The high-voltage battery contains dangerous levels of electricity, so exercise extreme caution and ensure all power is safely disconnected before proceeding.

There are You Tube videos on removing the battery, but if you watch some, you will see how to gain access to it and remove the covers required to access the contactors

T1 Terry

 

It could be a thing to do at some point, but right now it is premature and is almost certainly a distraction for the OP for the reasons ChapmanF mentioned in post #40