Hello?
I am not a reproductive specialist. But I think your vet can ask their lab services rep about progesterone checks or go get an ultrasound or refer to a surgeon for help. These cases can be frustrating but it is really important to make sure a testicle was not left behind in the abdomen.
If it wasn’t ingested you have nothing to worry about. The concern is when it’s ingested – depending on the individual dog’s tolerance and the amount ingested it can cause kidney issues.