Marking problems with Alpha dog

[COLOR=Navy][FONT=Arial]My husband and I own two delightful 8 lb Bichon Frises. Alex and Nikki have gone through advanced training and are very obedient, fun loving, sweet boys. They are trained to use potty pads in designated area of the house and do great. Alex, however, marks certain places when he is alarmed and we are not there to give a command to settle and tell him it's ok. He actually wants to see us check things out b/c he looks to me as THE ALPha. We have tried all natural downers that have helped a little. We have worked so hard to understand the whys. He is getting better as he is now 2 years old. Nikki is 6 months younger. The two get along great and Nikki respects Alex as alpha. I have owned other breeds and this is not something I have experienced as a continual problem until Alex. Any thoughts? :confused: