'Overt' means obvious. Overt behavior is literally observed behavior, but does have other contexts and shades of meaning. The reference usually means obvious behavior, or behavior which is considered misleading, because it's so obvious, depending on usage. The implications of these meanings are very different. The literal meaning is that the behavior should be taken at face value. The negative inference is that it shouldn't.
Examples of Overt Behavior:
|The premises are owned by a company.|
Breaking all the windows on someone's car is overt hostile behavior, in anyone's language.
Being so overtly nice is just plain suspicious.
Do you trust someone who is overtly so stupid, or not?