If you walk a field of corn long enough, you will likely find a few stalks with one or more leaves that show the characteristic signs of feeding by corn borer larvae. It's distinctive because it is usually a series of precise holes often called 'shot holes.' It happens that way because the larvae chew through leaves while they are still in the whorl of the plant.
What you're not likely to find is widespread feeding. Bt corn borer resistance trait appears to be working nearly two decades after it was introduced.
European corn borer has typically been more of a problem even before GMO corn in western states. However, pricing policies by some companies have favored planting of hybrids with the corn borer protection trait even in the Eastern Corn Belt, where outbreaks of European corn borer used to be more sporadic anyway.
The population of corn borer in eastern corn-growing states has dropped so low that some universities no longer track corn borer numbers. For years Purdue University entomologists would do trap counts and issue reports each fall of the expected size of the overwintering population of corn borer adults. That practice was dropped a few years ago because the population is so low that the size of the corn borer populations for the next year is no longer a big issue of concern.
About those few plants that showed corn borer damage that you found in the field- that's not necessarily a bad thing. Remember that you're supposed to be planting refuge acres where the corn doesn't have Bt traits. The goal is for some corn borers, in this case, to survive to help cut down on the odds that a resistant corn borer will develop that could feed on Bt corn.
Although only a lab that runs DNA or a super-sharp corn breeder who can recognize differences in hybrids would know for sure. It's likely the plants with damage were refuge plants. That's possible as long as you met your refuge requirements by planting a hybrid with refuge- in- a-bag technology. In this system, either 5% or 10% of non-GMO corn is mixed with GMO seed to serve as the refuge. So you have scattered plants throughout the field that are susceptible instead of a whole block of non-GMO corn.