Mere acquiescence or failure to complain does not always mean that the conduct is welcome. A hostile environment is created when the individual views the environment as hostile and it is reasonable for the individual to view the environment as hostile. Generally, a hostile environment is created by a series of incidents. You know the person's intentions and personality. There is mutual respect and rapport.
Thoughts associated with flirting are positive and welcome.
Uninvited sexual comments and suggestions. Grabbing, pinching, cornering, patting. If you are the person who is doing the flirting, you had better make sure the object of your flirting is aware of your intentions and that there is a well established feeling of mutual respect and rapport between the two of you. Better yet, save the flirting for the nonwork setting. Talk to someone in authority.