Alexander Summers had previously stationed a few people outside the Graves Residence to take turns watching Cherry, so when Cherry was kicked out, he received the news immediately.
He wasn't interested in interfering with the couple's quarrel and instructed his subordinates to continue monitoring, but didn't expect a heavy fog to roll in that night, causing his men to lose track of Cherry.
Someone's gone missing, so naturally, they have to find her.
Alexander put precautions in place against Cherry because he didn't want her disturbing his current life, stationing guards at the train station, airport, and port.
But keeping watch indefinitely was not a solution; how could it be as reassuring as having Gavin Graves help him keep an eye out?