When a bean crosses because of bee activity, it is normal for 1 or more beans in a single pod to be pollinated by foreign pollen. Usually, the cross will be undetectable at the seed stage though in a few instances the seed coat will be a different color.

The best way to detect a cross is when the seed are grown out and you find either an unusual vine color or other traits such as flower color that do not match the parent.

I have Striped Bunch beans growing in my garden with about 300 plants in a row 120 feet long. There are 3 plants with purple stems indicating crossing when I grew those seed. Striped Bunch has very unusual runners so it is easy to walk down the row and pick out the odd plants.

DarJones

A few years back I found one odd colored bean in a pod of Mr. Tung beans, the only other bean I grew then were my italians. Although these beans were grown about 20 feet apart with a greenhouse in between a bee must have found it's way from one to the other.
Mr. Tung beans are a dark beige in color, the italians are beige with purple swirls, the oddball is more pink than beige and a slightly bigger and a different shape, fatter. I've never had this happen before or since.
Here's a picture of Mr. Tung with the oddball. I just might try to grow it, see what it produces.