Day's family confirmed Thursday it was his body cops found in his Phoenix, Oregon home back on April 4 ... which is highly suspicious and mysterious, because cops had previously searched the home and found nothing.

The 76-year-old went missing last July 15, and there was an extensive search, which included combing his home. Police say it's unclear how long the body had been there, or how the body was missed during previous searches. They're not saying if there were signs of foul play. They had not been able to identify the body back in April because the condition of the remains was severely degraded, which rendered DNA and dental records unusable.

Dennis was originally reported missing by his partner of more than 45 years, who lives at a care facility. He says he got worried after Dennis hadn't visited in weeks.

Authorities say Day's family didn't find out about his disappearance until 6 months later when they saw a report on the local news. Adding to the mystery ... his car was found about 200 miles away along the Oregon coast with 2 strangers in the car claiming he'd let them borrow the car.

State police impounded the vehicle and continued investigating for a cause of death.

Dennis was a 12-year-old actor when he first signed on as a Mouseketeer in 1955.

As an original member of the Mouseketeers, he reportedly modeled prospective costumes for Walt Disney and appeared in publicity photos for the TV show. He last appeared on the show in 1957. He tried his hand at theater in NY and was later a drama teacher in California.