Unattended

If something is left unattended, no one is actively watching it and they probably should be.

Example

#1) She left her child unattended and the child was kidnapped.

#2) He left the stove unattended and it burned down the house.

#3) I left a campfire unattended and it burned down the forest.

#4) She left her running car unattended and it was stolen.

#5) Don't leave your dog unattended in the park.

#6) If you leave your bag unattended, it will be destroyed by security.

The opposite, attended, is not commonly used as an adjective. However, I person may attend to something, and this is similar to taking care of something.

Example

#1) I can't come with you now. There's something I should attend to first.

#2) Forgive me for leaving early. There's a problem I need to attend to.