Apt looks for a pgp key for every repo in the list. This is a securiety thing. That is the way it verifies that the repo is actually what it says it is and came from the source it claims to have come from.
When you do an apt-get update, which is what synaptic is actually doing, you will get an error on any repo that won't verify or doesn't have a key on your keyring. Apparently you somehow removed some/all the keys so that is most likely why the errors popped up.
You can ignore the errors and still use those repos but you now can't verify their authenticity without reloading the pgp keys for them.
I hope this helps you understand what is happening here.