Fortunately Yellowjackets do not re-use old nests, though it is possible that an exploring queen will find the area attractive as a site for a new nest by coincidence. Queens only establish new colonies in the spring; sporadic wasp sightings in the area later in the season would merely be foraging workers, and they might be drawn to an area if pest insects that create honeydew (such as aphids and scale) are present on any plantings near or above this part of the wall. Of course, if you see workers repeatedly entering and exiting a particular spot this summer, then that implies a new nest is located within the wall.

There isn't a practical way to discourage or repel queens that we are aware of that's backed by research. Baited wasp traps are only going to entice workers, not queens searching for a site to begin a nest. Filling the gaps with a sealant of some sort may work, but may also change the appearance of the stacked stone and so would understandably not be the ideal solution. If a colony does establish and is problematic, you can opt to treat it with insecticide dust (in a product labeled specifically for wasp control) that can be puffed into the nest entrance; utilizing professional pest control specialists for treatment/removal are also an option.

https://extension.umd.edu/hgic/topics/yellowjackets

Miri