Captain reported the air transport of undeclared Hazmat due to loading crew procedural errors. Reporter further stated Hazmat errors are a repeated occurrence at this location.

Narrative

During loading I asked the loaders from [company name] 'is there any dry ice or other Hazmat?'. They said 'No; none.' At this point I instructed them on how I wanted the weight loaded in the plane and turned the bulk of my attention to completing paperwork and weight/balance. The flight was conducted normally and without incident; however when the aircraft was being unloaded I overhead one of the unloaders say 'that's a lot of dry ice for one box!'. This is the third time in 2 weeks that [company name] has tried to put undeclared Hazmat on my aircraft. I was able to spot it the other two times but missed it this time. I have tried to politely and gently impress upon the loaders the importance of alerting pilots to dangerous goods being loaded on the planes but it still seems like a 50/50 gamble especially at the outstations on whether or not they will comply with reporting requirements.[This was recognized] During the offload when a loader commented on the dry ice.I think the loaders and sort centers at the outstations just don't care if the Hazmat gets reported or not. All they care about is getting the cargo out quickly. I have explained the importance to them and then one or two days later had the same conversation with the same people when I spotted more undocumented Hazmat. Not all the undocumented Hazmat has been dry ice. That is however by far the most common thing to fly under their radar and is the only thing to make it past me so far. At this rate though with how busy the loading process is and becoming darker on the ramp; it is only a matter of time before other hazardous materials are missed.I once again told the ramp manager that it is imperative that pilots are alerted to all Hazmat. I then told my supervisor about the event and am filling a [Safety Report].I think someone who has a managerial title needs to remind [Company name] of the possible fines that could be imposed on both companies for not following proper Hazmat reporting procedures because they do not seem to care about what the pilots are telling them.