I'd just say story writing.
But on a second thought they might have thought of some possible shielding technology or similar, given the fact it's the ship of a known smuggler and not just some standard-issue escape pod. But at the same time, they didn't check for hidden caches or anything like that either. So I'd just assume they've had the idea to look for someone hiding in there, but they didn't expect them to hide that well (or Han was simply too smart for them). After all it's obviously been a hideaway officials haven't found yet (while smuggling). Actually, not sure about this part yet, I think they leave their hideout before they're finished searching the ship? That case I'd just stick to "they wanted to make sure there's noone hiding".
Oh, and not to forget one thing: They had no reason to assume the ship is empty. In fact, they obviously were sure there's someone in there. The escape pod could start on its own, but the Millenium Falcon obviously couldn't. Also keep in mind we don't know whether they scanned the ship. They probably scanned it and even knew someone is in there, but they had to have a closer look onto where they hide (and that's the part they storm the ship).