Sorry to correct you, but muggles can and do see patroni. It's the dementors they can't see. If they couldn't see the patronus, than it wouldn't have been a problem that Harry cast the charm in an area full of muggles and in front of a muggle. It's only because they COULD - and DID - see it, that Harry got in trouble.
That, and the Minister is a slimy git.
-s.a.c.
Author's response
Thanks for your corrections... it's much appreciated.