That is not a separation anxiety, that is called "a pack unity sense". He must have known your trainer for quite a while, and, because, I guess, you might have done a lot together, Titan decided the trainer is a casual pack member. Young wolves and wild dogs often swap the packs if accepted in order to steal young females. In order to be accepted they make friends. I believe, Titan decided that your trainer is such a friend, because he likes him personally, as well as that person who visited your home. You should note it for youself - Titan is in need of a friend from outside of his own pack. Normally such male friend would lure a young dog away from the pack somewhere into the wild just for the sake of having a companion in case of meeting some enemy.Titain has read your trainers move as a departure and was keen to join him, that's why he barked. The same story repeated at home. This behaviour is pretty innocent and you shouldn't worry about, because it happens only before 3 years of age, unless you meet some stray male who run away from his home. If Titan likes him - he will follow him, so in any such situations your recall will save you a trouble.
A sense of a pack union is instinctive, and that is one of variations how new packs might form. It doesn't mean that here is anything wrong with his original pack in which you play a part of a parent. As you say, this sort of behaviour could be observed when you leave, and it reveals much stronger reactions. You should train him to be patient.
Tether him to the tree and ask him to sit. Walk away slowly, and return quickly every time he gets up, repeating "Sit!". At one moment it would be interesting for him what that is for, and he will keep in sitting position. Then introduce him to a new command, and pronounce straight after old command:"Sit!-pause-Wait!". Later in training "Wait!" should work on its own. Start making circles around him, come back to him and reward for being patient. Make the distance greater with time if he sits well longer than 12-15 minutes. Train him to wait patiently off leash. When that seems good - start hiding behind trees, building corners, anything you find suitable, where from you can watch him. Keep a written record on his progress. I always advise to everuone whom I train to start a diary on his/her young dog, without it it could be difficult not only to estimate his progress, but evaluate yourself as a trainer and to avoid doing one and the same mistake. It would be nice if his record states something like 35-40 minutes of wating in sitting position after 3 months. Telling him "Wait!" when you leave the house would install a sense into him that he is doing something in your absense, doing some job. But, please, don't start with it at home, he must grasp what is required outside home first. The command "Wait" could be very useful in many live situations.