It's always a relief when my DS 38 & DD 40, have 'senior moments' so I know I am not 'loosing it'.
Trouble is with so much of the world at our finger tips, internet, easy interaction with family and friends, being bombarded with information, our brains are constantly multi tasking. I know the answer is to concentrate at all times on the task in hand, but that would be boring, well except for driving. I do sometimes go off on auto pilot and so does DH until the other one asks why did you choose this route?
EVERHOPEFUL I use your technique of returning to the place where I lost the thought and it's still there waiting to drop back into place. It's magic how the brain works sometimes.