My old unit wants me to attend a layout because they can’t find everything, I’m no longer signed for anything. Do I have to go?

Even though you're no longer signed for anything, doesn't mean you can't help a brother out. If they can't find it, but you know where it was last seen....why not throw them a bone?

No, but if you can help and not burn bridges it is always better.

You should tell your old units to contact your supervisor in your new unit if they want a piece of your time.
If you have left the service then there are multiple reasons why you should not go back. To start with, you will not be covered medically if something falls on you and you get injured.