Normally two different species do not cross pollinate readily, however there have been some commercial efforts to produce interspecific hybrids and the hybridization barriers can be broken down (see this article). This is how genetic material can be shared between species. Just because you think you have pure open pollinated stock can only be verified by planting and seeing what happens.
The season can also be important. As a practical example I have grown Cranberry beans about a hundred feet from Kentucky wonder (both P. vulgaris varieties, so somewhat likely to cross) and for about twenty years had no problems with crossing - the resulting dry beans are quite different, KW is a plain light or darker brown bean and the Cranberry is purplish with flecks. This year there are signs of crossing, with about 5% of the Cranberry showing light brown with flecks. Curiously the KW does not show any visible signs of contamination from the Cranberry, the beans there appear quite normal, but who knows, surface appearance is only one characteristic of many possible.
This year, in this area at least, there were an unusually large number of bumble bees, and it is possible that their numbers and efficiency in pollinating flowers has had an effect. A hundred feet is nothing for a busy bumble bee. So next year I will be planting from older stock, use one variety only, and rogue any that show bad behaviour such as climbing when they are not supposed to, to try to keep the resulting beans pure.
If keeping the race clean is a high priority and you have many insects then plant your beans in alternate years. They maintain germination viability for several years. Also choose varieties that are distinctive in some way so that it becomes obvious when they deviate from normal appearance and habit.