It means that the person is thinking about ways to cause trouble; in other words, they are mischievous or a practical joker. Someone who is thinking about their own troubles and problems is called...
Absent-mindedness is often caused by failure to pay attention to details at the time they happen. It can also be due to age or other issues that affect the brain's functioning, such as attention...
Abenst minded means that they are extremely forgetful, or wasn't thinking at the time or something similair. It is just an expression that a lot of old people use.