First if it’s a cold day of course we all tend to put our hands in our pockets.
If you go to a seminar and his hands are in his pockets there could be a few reasons for this. It could be they are nervous, anxious or uncomfortable with the circumstances. Even when someone is bored or just not interested hands linger in the pockets.
If you find yourself putting your hands in your pockets to much it can send a message you lack confidence. If you want to give a good impression avoid this action.
The most one to be wary of it could be that they are trying to hide something from you; this would not be a good one if they are stuttering or uming and harring alot in their conversation with you.