He may have shifted suddenly, or done something else that you didn't see.
Yes, absolutely he could have moved in a way to cause that reaction. When training watch dogs, one of the things that you do is have a person act suspiciously. They move sideways, drag a foot, hunch while moving, etc. Some dogs become suspicious at just a halting step, some require an extreme caricature of a thief shuffling away with a sack over their shoulder.
The 2 things that I find so strange are:
1 - That she was okay with him at first.
2 - That she reversed roles with Missy. That Missy was fine...better than fine...while Chelsea wasn't, is just so weird.
I'd be bugged, too. And, you know, Tony could be right... I'm sorry that it ever happened. No one needs any other problems.
Love All Life, Thank You For Posting! :)