Joe Jackson -- SQUATTER

Joe Jackson has been living at the famous Jackson compound in Encino recently ... but no one knows how he got in ... and it appears he's a garden variety squatter.

As we previously reported, Joe was seen entering the home today ... and yesterday, it appeared the home got a visit from some medical personnel ... presumably to perform some tests on Joe.

The house is owned by the Michael Jackson Estate. Katherine Jackson has no legal claim to the property.

Sources tell TMZ ... the Estate is paying $30,000 per month for the Calabasas home where Katherine and Michael's kids have been living.

The only way Joe could be living in the Encino house legally is if he got permission from the Estate ... and sources connected with the Jackson family tell TMZ ... no such permission was ever given.

Sources tell us ... it ain't the Ritz because the house is currently undergoing extensive renovation.