I agree with everything that Petra's Mom said. Great advice!
My dog Albert had had a similar problem when we moved to a new city. The first day at the new location he had an accident indoors. The same happened the first few times I took him with me to the pet store.
I have found with Albert and several other friends dogs that when some dogs are potty trained they learn not to pee indoors at THAT home/location and assume that anywhere else that is not in THAT home is ok to potty at. So their first few times at a new home or location they may have an accident. Since your dog has had accidents indoors at your home I would be sure that your dog does business before you go indoors at new locations that he is not yet familiar with to avoid the accidents altogether.
If he does have an accident just be sure that you immediately let them know that what they did was not acceptable (but don't scare the dog because it can make them release more due to fear) and immediately take them outside and tell him to "go potty" or whatever term you prefer to use.
When my dog Albert was still a pup I used the same technique that Petra's Mom talked about. When we went outside I would tell him "go potty" several times until he went. When he did go I would praise him by saying "Good boy, you went potty. etc." and sometimes rewarded him with a treat so that he would want to potty whenever I said the word.
Good luck!