Do you think someone has emptied a bottle of bleach on your plants? It does kill off plants. The tree may be OK. I would be tempted to soak the area with a hose to dilute any remaining bleach.

Surely if someone had soaked your plants with bleach, you'd have smelled it, wouldn't you, back when you found the empty container?

What was the shrub that died, and which tree is it that's looking a bit sick?

How big was the container? Also, how close was your shrub and tree too it? If it was more than ten feet then it was just a coincidence and not a cause.

My shrub was a Mexican orange blossom, Choisya Aztec Pearl and usually was beautiful in Spring. Ive had for years. It was planted where we buried a much loved cat some 20 years ago. It had been looking a bit poorly, but I thought it was all the spring rain, I didn't smell anything but I have sinus problems. Im afraid I don't know what the tree is, I'm not very good at identifying them, but it has great birdlife. I will check it out, when it stops raining and get back to you.

Is there any dead wood in the area, such as an old tree stump or remains of a woody shrub? Did you notice any honey coloured toadstools anywhere in the area during September, maybe even now?