EXCLUSIVE

Stan Lee might've allowed way too many hands in the cookie jar, because another $1.4 million just up and disappeared and he has no idea how ... TMZ has learned.

Sources connected to Lee tell us ... detectives working in a specialized unit that deals with elder financial abuse showed up at Stan's Hollywood Hills pad Friday to look into the latest theft. We're told detectives met with Lee for more than 2 hours.

Detectives peppered Stan with a bunch of questions ... including exactly who had access to any of his accounts and a timeline of when the cash went missing. Stan turned over financial records to help them find the culprit.

We broke the story ... someone siphoned around $850k out of one of Stan's bank accounts to buy a condo. Another $300k also disappeared.