How do these vessels routinely get through, and they have very little stealth tech? If the Spanish / NATO / US military ignores them or can't see them, doesn't that mean actual (enemy) Chinese / Iranian / Russian subs can get away with a lot by pretending to be simple drug smuggling boats?
Maybe the military lets drug smugglers operate, and even ignores foreign governments doing submarine subterfuge like bugging undersea cables or dropping spies / equipment onto shores at night? The purpose of turning a blind eye to be protecting information about anti-sub capabilities, if the drug kingpins or enemy spymasters don't know which of their missions successfully maintained secrecy? Maybe when the anti-sub network alerts successfully, they watch the spies and equipment that lands from a distance with high-tech drones / satellites (or heck, a simple patrol squad with night vision binoculars), follow them for a few months, then tip off the cops (if it's a mere drug cartel) or send counterintelligence to arrest them (in case of actual spies). So the enemy has to guess whether they got seen on the submarine landing, or were compromised by some other means.
That these boats can get through might be a catastrophic failure of anti-sub tech, or it might be part of the normal I-know-you-know game of international espionage. I suppose we civilians wouldn't be able to tell the difference. Perhaps the biggest clue is that drug subs have been a thing for many years, but there's a notable lack of public alarm from military / politicians over it. Given that, I'd put my money on the hypothesis that the military knows exactly what the smugglers are up to, but chooses to ignore it to keep anti-sub capabilities and weaknesses secret.