Red Triangle problem

Dear Prius community,

I just bought a 2007 Prius, [package 6 if that matters] with 42k miles on it. I was driving it the other day and this check engine light came on..I didn't know what the deal was so I went to my mechanic who doesn't usually work on Prius but he did some work before on a Gen I Prius...he pulled some code on there and told me that the inverter water pump is probably gone bad after 42k miles. So he replaced it with a new water pump. Then he removed the check engine light with the scanner tool.

Then one day later, I was driving on the road and this red triangle came up on the dash. I freaked out. I called my mechanic guy who did the water pump and he told me the inverter or hybrid battery must be going bad..and to return the car for a refund since it hadn't been 7 days since I bought the car..he said I was legally within my right to get a full refund.

I did some research on the net and on this site about all the cases regarding this red triangle and what you people posted is not good. So naturally, I became worried. Most of all, I am worried that it might be inverter or hybrid battery related.

So I went to a different mechanic in the morning who specializes in Prius. When I started the car up in the morning, the red triangle was no longer there..and during my drive to the shop the red triangle didn't show up. But I went in to get it checked anyways to see if it would throw off some code.

I sat next to the mechanic at the shop and there were NO codes being given to the scanner tool. It was blank..nada.. so...we opened the hood and saw there was little to no coolant in the radiator..which was odd because I had just poured some in the other day when the first check engine light came on. The guy I bought the car from told me to get some pink Toyota Long Life coolant because he thought that the coolant system had air in it and had to be bled. So I added some coolant in there.

But as the mechanic and I looked at it..it was empty dry. I poured 3/4 of a gallon of the pink coolant in the car today..I had the engine on and the air blowing hot inside the cabin..I kept pouring coolant for about 1/2 hour..it would go to the top..and then go down..and I'd pour some more..repeatedly. I then heard some gargling sound from the dash board..I was told that indicated air was still in the system. But to keep putting more coolant in as it the coolant level dropped. Was I doing this right?? What about the bleeder valve? Do I need to open up the bleeder valve?

I then proceeded to test drive the car for about 50 miles..no red triangle. It appears to be gone.

But the question remains:

1) What do you think caused the car to light up the red triangle?? Was it because of the coolant level being low? Or because there is air in the system?

2) Am I bleeding this thing properly?...and how much coolant does this car take?

3) Is this a common thing for Prius? at 42k miles?

4) I also found this plug that was not connected...located right next to the hood lock..don't know what it is..do you?
[See Photo]

5) My first mechanic said to take it back due to what he thinks is an inverter or hybrid battery related problem. He said that the red triangle was specifically for electrical issues and not due to the coolant issue. Is this right?

I heard alot of good things about Prius which is why I bought one. I don't want to lose confidence in Toyota because they do have a decent reputation of making good cars..but I need to get some answers.