When the FBI shows up, they cut the power to the building, meaning the electronic gates in the garage are now frozen in place. Hans would have to plow his ambulance through the bars just like Argyle in order to leave, and we're willing to bet one of the 100 or so cops on the scene would notice something like that.
Even if he manages to get out of the building, Hans is far from home free. The whole point of the terrorist song and dance he feeds the FBI is to disguise the fact that he's really just a bank robber. According to his plan, blowing up the roof will lead everyone to believe that he and his extremist buddies detonated themselves and their hostages, because that's the kind of silly bullshit that terrorists do. However, the bomb doesn't destroy the vault, and Hans never intended it to. Meaning he purposely left a giant, empty safe hanging open for the authorities to find.
"... just leave a note saying it was like that when we got here."
It won't take them too long to identify the charred bodies, particularly when they already know exactly how many hostages were in the building. When the FBI realizes their count is short a few terrorists and about half-a-billion dollars, they're going to know that Hans took that fucking money. At best, he's bought himself an extra week, and then Interpol is going to burst into his Majorcan hotel room and drag him out through the lobby in his bathrobe.