Possibly seperation anxiety

Question
I got my dog from the humane society about 7 months ago.  He is mostly Rat terrier, but possibly has some chihuahua in him.  He has done great with the family.  He just has one problem, when I leave the house he poops on the floor.  We know it is within 10 minutes after I leave.  He doesnt even care who else is home, he just goes.  I make sure he goes to the bathroom outside before i leave for work and also take him for walks in the am. I feel like I need some advice on what else I can do for him.

Answer
Carissa,
Yes, I believe you could be right, it could be separation anxiety or it could be a learned action from a long time ago.
Here is what I would like you to try.  Every time he goes out side, you tell him  "GOOD POTTY" and give him a treat.  
On your next day off,  I want you to leave like you always do, but I want you to wait one minute and come back in.  If he has gone to the bathroom in that minute  bring him over and show it to him.  Tell him "NO POTTY" very sternly and bring him out side.
If he hasn't gone to the bathroom you tell him  "GOOD BOY" with a lot of petting and a treat.  Wait five minutes and go out again, but this time wait outside for 2 minutes and repeat the process.  If he went to the bathroom then back down the time a little.  If he didn't go, then increase the time.  Do this all day long and keep increasing the time.  It would be good if whoever is home could watch him and catch him in the act and say "NO POTTY", but if not you can still work this thing yourself.
When he finally gets the idea you tell him good boy and take him for a walk or a ride in the car, that is the best treat of all.
Get back with me on this and let me know how you do.
nancy