We've been noticing Simon limping lately, so we took him to the vet. After x-rays - he's got that small dog (30 pounds is small?) condition called trick knee.
Basically, his right knee cap dislocates to the inside of his knee. He does not seem to be in pain, but he does limp stiff-legged until it re-locates to its proper position.
He'll need surgery to correct the condition which could be congenital or due to trauma.
Just another day in the life of Simon the Sad!