What actually happens at 80-90 security?

I'm going through Shido's Palace atm and I purposefully raised the security level to try and find Treasure Demons. When I enter battles Futaba's saying stuff like "They're onto us!" and "A total lockdown this battle will never end!" The latter thing is particularly confusing to me - is there a chance that if you enter a battle and fight, more enemies will keep spawning into battle endlessly or something? I don't see anything clarifying this on the megaten wiki so.