outside wall frost build up

[masterrace1]

have a typical walk in set up, with walkin cooler and inside is the door to the walk in freezer. the unit is operating properly and maintaining temperatures. the problem is that we saw frost build up on the exterior outside wall over the winter(as it is butted up to the wall). What would cause this?

[monkey spanners]

A gap between the panel joints.
Faulty or damaged insulation in the panel.
A hole in the panel.
If it is around the door, the heater tape could be faulty.

Basically anything that lets the cold out.

Or you are in Canada, its winter, and it would have frost on it anyway