My Bullmastiff is just over a year old and 100% housebroken. But, upon trips to Petsmart and friends' houses, he marks inside when he knows to go outside. What can I do because its really embarrassing and I don't want to stop bring him places with me as he is my "little" buddy :). This problem has developed very recently. He is also not neutered. Thank you!

He is marking his territory, this is what male dogs do who are not neutered.

If you want it to stop the best bet is to have him neutered.