Concrete transitions are gaps or seams where two construction elements meet, (i.e., where pavers meet foundation, curbing meets gravel, etc.).   Because concrete is expansive, these gaps tend to offer critters a fortified resting place in which to hide during the day.   You’ll often spot scorpions relaxing near these gaps early in the evening when they first emerge from their daytime rest.  

Be sure to approach them quietly though, because skittish runners will often dart back into the inaccessible gap at the first sign of trouble.  If this happens, however, come back again the next night and you’ll often see them again.