@S-2 If the guests are repeatedly breaking your House Rules - whether about parking, or unpaid guests/visitors - even after being told to stop, this alone is grounds to terminate the booking and remove them from the property.
If you're unsure of whether a specific rule has been broken, either you or a trusted local contact can create a pretext to go to the property and have a look around. If you plan to enter the house, the guests should be informed of this first.
But I would not bring up your suspicions of criminal activity if you don't have solid proof of it. If people are coming in the middle of the night and leaving after a few minutes, that is definitely suspicious, but if the extra cars are staying for hours or all night it sounds like something else.