Banners are visible through blocks like the honey and slime blocks or water.
In order to replicate: Put a banner on land, close to water. Hop into the water. The banner is visible as though you have X-Ray. Alternatively, place a banner on land, place two honey blocks in front of it and stand behind the honey blocks. Same effect.

Attachment below if I didn't explain it well enough. (the player on the screenshot is in water.)