Passive Perception or Active Perception Check to detect hiding enemies?


I’m DMing for a party that constantly says stuff like we’ll walk slowly and carefully, or we’ll look around as we walk, to make sure there’s nothing threatening or out of the ordinary.

Say there’s a monster hiding near the PCs (stealth check 14). All the PCs have a passive Perception lower than 14.

Given the above statement by the PCs, should I now ask for an active Perception check to see if they spot the monster hiding? Should I always ask for an active Perception check from now on, since they’re always being careful? On one hand, they did say they were being careful and looking at their surroundings. But on the other hand, I feel like that’s already assumed, as a party of adventurers wouldn’t just nonchalantly walk into a dungeon – therefore I feel like this is exactly what passive Perception is for.