I've been dealing with this also but I used 3M outdoor tape just below the corner on the flat surface and it's good as new. A new J clip may not be my fix since tab on actual bumper isn't as strong as new but my corner looks absolutely factory fitted now with a free fix essentially

 

With my 2010 RX 350, the front bumper was sagging ever so slightly and at the sides, where it meets the headlight housing, it was more noticeable with the clips visible. I would push it back in, or try to but it was still noticeable. I purchased the car CPO and it had no previous accidents that I was aware of and nothing was noted on the Lexus service history. I certainly hadn't had any accidents.

I hadn't noticed it until I tried to sell the RX 350 just over a year ago and one guy and his buddy who were looking at the car. They were convinced the bumper had been replaced and pointed out the spots on the corner where the bumper didn't seem to fit right. I knew they were wrong but I could see what they were pointing out and it's puzzled me ever since.

It was about 6 weeks ago when I actually did end up selling it to buy my 2009 LS 460 that it dawned on me where the damage came from. The sagging was from my garage door closing on the bumper and then going back up once the security safety feature kicked in. It applies just enough pressure to punch the bumper out of place. This happened a handful of times over my 2 year ownership and while it had left very minimal paint damage to the car, the sagging front bumper was almost certainly from this. I happened to actually witness it a couple of times and the collision is actually more violent than you would think and sounds pretty bad. I was so concerned with paint damage that it never occurred to me that the bumper would be pushed out of place. The garage door would catch on the bumper or on the license plate holder. It would put just enough pressure to push the bumper down, and then go back up.

I'm not saying that's what's going on here but it certainly is possible.

 

And to the OP, I see that you pull into your garage and your rear bumper looks to be pretty close to the garage door. It could be that you punch the garage button on the wall before going into the house and don't stop to see if the door pops back up but when you go back out to the garage, the door is mysteriously open?

And with my RX too, I could push the bumper up and the fitment would be perfect but it wouldn't stay there and it would sag back down 1/8"-1/4".

Just saying, this is certainly possible that it's garage door damage...if not from you from the previous owner.

 

Since I am the second owner, I would guess it is a possibility. But I think you are on to something. I now believe this issue is caused by putting weight on the rear ended, above the bumper, like resting luggage on it before you lift it into the trunk. I bought my car off lease, and it was a corporate car. The top of my bumper had many small surface scratched that tell me there was a lot of stuff in and out of the trunk. I will look further into this over the weekend But I bet if I put weight on top of the bumper, I will see movement on the spot in question.

 

Go up to your bumper and put weight on it where the garage door might hit it and see where it flexes and where the stress shows...then pull it back up to where it should be as if new. That's what I did and that's how I knew it was from something coming down on the bumper, like my garage door.

 

I have the same issue, *ever* so slight, with my LS. I didn't even notice it for awhile, but now that I see it it's kind of irritating me. The front bumper sags maybe 1/8-1/4 of an inch, only on the right side. It's tight against the fender next to the wheelwell, but the gap between the part of the bumper next to the grille and the hood is just a bit too big. There's no record of any damage to that part of the car in the service history (although there was a respray of the rear left door after the dealer scratched it during a service) and the VIN stickers are all still on the car. I've never taken the bumper off, so I don't know what's underneath -- I'm wondering if this means a retention bracket is slightly bent.

 

Some of the lines are like that on my front bumper but I know my bumper has been removed at least twice, once before I owned it as the right front of the car was painted, and once by a local body shop to fix the foggy clear coat finish from the first job. The lines aren't perfect but no one would every notice except me.

Perhaps you've got some bent brackets or clips. You could try having them all replaced but I doubt that's worth it.