Adopted Dog Peeing Inside at Night?

I adopted a dog about 3.5 weeks ago. They said he's only a year, but other body features are showing he may be much older, maybe closer to 5. I only know that he came in as a stray when I rescued him. For the first week to two weeks of having him, he did great! No accidents in the house.

Then I brought a friend over a few nights ago, they stayed the night as well. And the next day I started smelling something peeish. I thought it was my hamsters, but it turns out he peed in one area of my room. And he must have kept going there because I didn't realize it until it was too late. I have tan carpet that you can't tell if he peed or not, have to go by smell. I never did anything, told him no, because it was too late to do so.

Well the other morning I apparently wasn't fast enough to take him out (he is the first thing I take care of in the morning as I know how I get!), and he peed in the corner. I corrected that one because I saw. But now I'm afraid he is continuously going at night and it's in the back of my mind to the point I don't sleep well anymore. Any suggestions? He is fine when I am around him.