What signs are there of lairs nearby? Encountering a monster without encountering them.
Monsters should have real impacts on the world around them. The presence of a giant lair should never be a surprise to the party if they arrive there. Before you arrive at a human town you know it is coming if you pay attention. There is often literal signs and markers but beyond that you go from sparse farm buildings and large stretches of tree lines to having some residential style buildings. Why would your players not be able to find the signs of a basilisk before they suddenly get paralyzed? However, I believe that these should not be just window dressing you should need to interact and investigate for full information. For this purpose I am going to write a series using the 50 monsters of Monsters& for what you may encounter in the hexes surrounding a monsters lair without actually encountering the monster.