Both are grammatically correct and convey the same meaning. These would usually be spoken in a rhetorical sense to yourself. If you wanted to ask someone if they know where your keys are you would say, "Do you know where my keys are?" or "Do you know where I left my keys?" For example, if I am rushing out of the house and can't find my keys, I usually say to myself, "Where have I left my keys??" or "Where did I leave my keys?" as rhetorical questions. They are not meant to be answered by anyone - just a statement to myself to indicate to others that I have lost them and cannot find them. I hope this is helpful!

You could use either..but "where have I left my keys" is a little more formal. Personally, I think "Where did I leave my keys" sounds a lot more natural.

They're both correct. In this context, you could use either past simple or present perfect. A general tip: if you're ever unsure which of these two tenses to use, you should always go for the past simple. It's far more likely to be right, or at least acceptable. If you use a present perfect instead of a past simple, it's a mistake. Vice versa? Not so often.

Both are acceptable. I find myself often saying, 'Where did I leave my keys?', when asking somebody or even to myself. :)