You know I have noticed similar behaviors with Dallas as you have with Jack & have worried that it may be the start of separation anxiety. Dallas is also okay in the crate after crying for a few minutes after we leave. However, if we don't crate him [like when we leave for 10 minutes] he goes NUTS! If we are outside & he's in the house by himself he will pace between the front door & the sliding glass. He'll scratch & scream at both doors. He will start getting destructive, even if he can't see us. It's like he lashes out because we've left him. Also, if he even thinks that we're getting ready to go somewhere [like going through the motions of putting on our shos, picking up my purse, etc.] he'll start darting back & forth between us & the front door & whining.
One time when I was outside cleaning my car [boyfriend was inside with him!] he went into our guest room & managed to hop onto the window sill [which I still cannot understand how…] & just screamed at the top of his lungs!!! Mark thought Dallas had hurt himself so he rushes in & while going to get Dallas down, he actually jumped off the sill & landed really awkwardly on the ground. He was fine but I'm scared that if this continues he really will hurt himself! He is fine if we're in the house & in a different room from him & he even will sit quietly in his crate while we are in the apartment. It's just when one of us or both physically leave the apartment, he doesn't like it one bit.