When you forget a thought, where does it go?

All phenomena arise, last for a time, and cease to exist at some point. Phenomena occur in dependency on causes, and cease in the absence of causes. When a thought arises, it will cease when its causes and conditions are no longer present.

Another perspective:

After hearing of the science of neuroplasticity, but I would say that the memory actually sticks into a different place on your neural network. So maybe not as easy to recall it, but I don’t think it is entirely ‘forgetten’. Maybe not as prominent in your brain, but still present in another way.