I encountered an outdoor electric panel and wondered if this is allowed by code. It is exposed to the weather and has a missing breaker or slot cover.

[Jeffrey Moore]

Here in AZ, exterior mounted panels are the norm and not the exception.
An unfilled opening in the deadfront is worthy of a write up

[Christopher Currins]

Being that you're a home inspector:
http://asp.homecheck.com/search/type_detail.asp?id=22419


you should no we don't inspect to code. Also that any opening (missing breaker or slot cover) in a service panel is a safety hazard.

If you don't I feel for your past clients.

[Robert Ernst]

I don't know about code but I do know that there are service panels that are designed for the exterior. Usually there is a meter attached to them here. The door often covers the whole front. If your a Home Inspector join NACHI.

[Paul W. Abernathy]

If you have an exterior panel enclosure (NEC Article 312) it is required in accordance with Section 110.28; this section defines the type of NEMA rating that must be used on exterior rated panel enclosures.