You cannot be "forced" physically to undergo a drug test. However, if you fail to voluntarily take one, then absent an employment contract or union agreement to the contrary, you can be disciplined for not doing so (up to and including suspension or even termination). This is true whether or not you have left your palce of work. Basically, in an "at will" work relationship, a company can set the conditions of employment much as it sees fit (so long as no form of actionable discrimination is a factor).