Go look at the enforcement sections of the licensing boards. The boards are fining out-of-state firms for not having a COA even though the office is not in-state. My employer who is not an engineering firm has a COA in Texas and I personally have a COA in Texas also since anyone practicing as a sole proprietor has to have one also.
Under Texas law, PE's can only seal work done under their direct supervision which is defined by the board. The best definition I heard regarding this was by a lawyer who stated, "If can't eyeball the drawing preparation, you can't seal them." Thus sealing shop drawings prepared by a detailer in another state far removed from the location of the sealing engineer is not legal. I go through this on a monthly basis with others who are not familiar with Texas law.