EXCLUSIVE

The investigation into how Stan Lee's bank account got swindled out of $300k has gotten even more bizarre -- it appears someone also used the comic book legend's money to buy a freakin' condo.

Sources close to the Marvel Comics legend tell TMZ ... someone recently plunked down $850k of Stan's dough to buy a condo in WeHo. Neither Stan nor his financial team approved it. We're told the revelation came to light after his team rolled out a full audit following a recent check forgery for $300k.

As we reported ... Stan filed a report with Beverly Hills PD claiming someone forged the check as a loan from Stan's bank account to the merchandising company, Hands of Respect.