A carrier with a note on a hot day, it could have ended badly.

[Catherine]

A staff from Homeward Bound Cat Adoptions was just locking  up when she noticed two carriers out in the Las Vegas heat. It was a mama cat and her kittens, panting from the heat. She brought them into the cool building and they quickly recovered. A regular foster parent just happened to be there and she had room to take in the little family. Mama and babies have thrived and are ready for their forever homes.

https://www.thedodo.com/daily-dodo/littl...but-a-note

The cats had no food or water and were left out in the intense heat. If they hadn't been seen when they were, it might have been to late.
Instead of looking forward to happy lives in forever homes, their lived could have ended that day.

We can understand and accept that people sometimes can't afford their pets anymore. Shelters would be glad to help them. 
Just leaving them outside like that is irresponsible. Too much was left to chance. Even an hour more outside might have been too much.
If you have to give up a pet, please do it responsibly.