I have had several buyers throughout my career tell me that they didn't need to bother with a showing of a certain home as the back door was open, so they went in. This terrifies me for a number of reasons!
1) It is Trespassing - and it's Illegal!
2) Does the home only appear to be vacant?
3) What if the home appears vacant - and the owner has a gun? There are a lot of gun owners in Arizona!
4) What if it is vacant & there is someone there much more desparate to be there than you?
Please be safe - contact your agent or the listing agent to schedule a showing - don't try to find a way in yourselves!