(CAPS is for highlighting) A more likely interpretation,though of course still needing confirmation .
When Satori puntualizes to Reimu that " However, we don't know if she ( Marisa )is being controlled by it ( the Eternal Criminal Evil Spirit ). Be careful not to end up being possessed " it is not meant to signal Marisa as a possible accomplice,although she could still had been too "researcher" when first dealing with it and ended up being " The Main Carrier ", but rather the Evil Spirit IS ABLE TO SWITCH BETWEEN ACTIVELY Controlling its Possessed Victim and HIDING while possessing one without an active control , THUS at the end Satori is warning Reimu that it is AT THE MOST DANGEROUS when just hiding since it has more chances to possess by surprise(by hiding in any drink which act as a medium for it),to camouflage itself and to cover its escape.