Previously Trained Adult Dog Peeing Indoors?

We have two Pomeranian dogs (males). The older one, 3 years old, has been previously trained and hasn't peed in the house since he was a puppy. But recently he has been peeing in the corridor near the doors whenever we go out. Now, whenever we go out I put him in the cage, with food and water. It's been going on for a month now, but when we don't put him in the cage he still goes to the doors and pees. And the pads on the floor in the cage are full of puddles of pee.

Downstairs, near the door, it can be more than one puddle. We suspect several causes.
One of them is that the dog is mad that we left the house for a week and left the dogs with my sister's family, and he missed us.
Another is that when we were away the dogs hadn't been out for a single walk.
And also we think that it may be something to do with the female dogs in the area, because he has started 'humping' our other dog (male). That one does not pee.

We are very frustrated and we can't find anything we can do to stop this. We have read on some websites that you should not punish the dog, but if I don't put him in the cage he will definately pee and if we don't let him near the doors he will pee somewhere else!
Please could someone tell us what we can do to stop him?

By Marzena from Torquay, Devon