Securing our Border Act This bill addresses issues concerning border security and immigration, including by transferring funds from the Internal Revenue Service to certain border-related projects. Specifically, the bill transfers certain funds previously appropriated for tax enforcement activities (e.g., collecting owed taxes and conducting criminal investigations) to fund (1) nonintrusive inspection systems along the northern border and southwest border of the United States, and (2) the construction of a border wall system along the southwest border. The bill also authorizes the U.S. Customs and Border Protection to pay recruitment, retention, and relocation bonuses, subject to various requirements and limitations. For example, a relocation bonus may not exceed 15% of the agent's annual basic bay and must be conditioned on the agent agreeing to serve for at least three years at the new duty station. The bill also modifies the treatment of non-U.S. nationals ( aliens under federal law) arriving by land from a country next to the United States. Specifically, if such an individual is not clearly entitled to admission into the United States, the Department of Justice must (1) return the individual to that neighboring country or a safe third country while removal proceedings are pending, or (2) detain the individual while the individual's asylum application is under consideration. (Current law authorizes DOJ to return the individual to the neighboring country but does not require such action or detention.)