In Prison Break season 5 episode 8, Michael, Lincoln and Sheeba come to the house at Ithaca where Sarah and Jacob lives.
After getting off the vehicle, Michael suddenly sees their post box named Ness 1990. Then suddenly Michael leans to the drain alongside the road near post box and says that's why Sarah didn't get any of his letters.
How did he know already to where to look at exactly?
In a flashback scene, after that, it shows that Jacob opens a letter every day and put Michael's signature origami swan into the drain pipe.
But how did Michael know exactly that Jacob did it? It could be numerous reasons Sarah didn't get Michael's letters, or maybe she did get them but didn't have any intention or chance, or doesn't know how to reply to those letters.
Is there anything I missed here or else how did Michael know exactly where to look at the first glance of the post box in front of Sarah's house?