Data transfer between EC2 AZs

The situation you're describing is unusual for a single EC2 instance running on the free tier with no Elastic IPs. Typically, data transfer between Availability Zones (AZs) occurs when you have resources in different AZs communicating with each other. However, in your case, with just one EC2 instance and three Docker containers running on it, you shouldn't be seeing data transfer between AZs.

There are a few possibilities to consider:

1. Misinterpretation of billing: Sometimes, the AWS billing interface can be confusing. What you're seeing as "data transfer between AZs" might actually be general data transfer in and out of your EC2 instance.

2. Unintended network traffic: Your applications (ASP.NET, Node.js) or Redis might be communicating with external services or endpoints that are located in different AZs within the same region.

3. AWS services communication: If your applications are using any AWS services (like S3, RDS, etc.), even if you haven't explicitly set them up, they might be located in different AZs, causing inter-AZ data transfer.

To investigate further:

1. Check your AWS bill carefully to understand exactly what type of data transfer is being charged.
2. Use tools like tcpdump or Wireshark to monitor network traffic from your EC2 instance and identify any unexpected connections.
3. Review your application code to ensure it's not making calls to services in other AZs.
4. Consider using VPC Flow Logs to get more detailed information about IP traffic to and from network interfaces in your VPC, but be aware that this service has associated costs.

If you're still unsure after investigating, it would be advisable to contact AWS Support for a detailed explanation of your charges.
Sources
EC2 regional data transfer - in/out/between EC2 AZs question. | AWS re:Post
Data transfer limit in free tier. | AWS re:Post